When he becomes Boyfriend/Fiancee Carmy (bc let's be real, they're already engaged):
Wrote this before Season 5 so some of this is already canon!
Canonically, Carmy likes to decide what Syd is wearing. 6 days a week for most of the day, he decided she would wear a blue apron pre-service (he doesn't care about the rest of the staff just that they are "dressed like Syd") and he gifts her a custom Thom Browne chef's jacket with her exact measurements he somehow guesstimated based on multiple surreptitious observations. I've got a whole meta on how he always notices what Syd is wearing. He has designed dishes based on her dress and bandana, complimented her outfit and kept her scrunchie; he also dreams about waking up next to her with her hair curly. So, as boyfriend, I think he'd have a lot of opinions and preferences about her clothes down to the accessories. Like, I fully expect Carmy has a favorite bandana, ranks the earrings and hair ornaments. Most boyfriends would be bored when their girlfriend wants to shop for clothes but I think Carmy would enjoy being involved and would even gift Syd designer clothes and jewelry they mutually appreciate.
Carmy is the same height as Syd and, in a gender-swap that is foreshadowed by the song which plays in Syd's apartment party "dress me up in women's clothes," he likes to wear her clothes (sweaters, sweatshirts, hats, T-shirts from her mother with black empowerment messages). He also keeps one of her scrunchies or bandanas in his pocket to allay his own anxiety when he has to be away from her for any length of time.
Once these two actually start sleeping together, they will become exponentially more annoying to everyone who has to interact with them. They already gesture and speak in synch. That's about to evolve into a whole repertoire of giggles, secret signals, code-words and PDA--initiated by Carmy. Richie has no leg to stand on because he and Jess are now officially a thing too. Nat is always annoyed by them though.
Carmy is very physically affectionate. He does not like to touch or be touched by most people. He can't keep his hands to himself with Syd. He either has his hand on the small of her back or bumps her shoulder (Season 2 canon) and his crotch and eyes are always angled towards her. People, especially other men, have to talk to Syd with Carmy physically in between them. Syd does not show PDA or act possessive in this way. However she is constantly doing special little acts of service for Carmy that make him blush with happiness and they both walk around in a constant glow that is obnoxious to witness, particularly for anyone unlucky enough to be single in their vicinity and even people in relationships whose partners never can live up to Sydcarmy.
When it comes to intimacy, Carmy is very private. He resents sharing Syd with his family. He doesn't want to share her time with anyone in his family when they are home together; he needs privacy and safety with Syd before he can relax into making love to her. Everyone has had to learn that they cannot monopolize her attention outside of business hours. He sets the boundaries on her off-the-clock time which she has not been able to do, like a good partner taking care of her wellbeing. Carmy also miraculously discovers he and Syd can sleep in and arrive at the restaurant later in the mornings if they prep the ingredients before. Syd asks him why he never thought of these solutions before and he sheepishly admits he wanted to maximize every second he could with her but now that they live together, he craves alone time with her instead.
Carmy spends quite a bit of time with Emmanuel. It takes Syd by surprise sometimes how much time her dad and her boyfriend spend together without her around. Carmy relishes the gentle wisdom and bonhomie that Syd's Dad exudes and hanging out with him heals the wounded place in his own heart that Mikey's death and the Berzatto Dad's abandonment left behind. Emmanuel has taken to dropping by the Bear in the daytime and even sharing Family Meal. He gives Richie great advice on how to win over Jess's Dad and her brothers.
Carmy loves watching Syd. If she is folding laundry or packing meals to take to her dad's place, Carmy enjoys watching her. He loves the sound of her voice and demands she keep talking to him when they are making love. Sometimes, in social situations where he feels anxious without Syd, Carmy repeats word-for-word things that Syd has talked about. He is the world's foremost expert in Syd Studies.
Syd has quite elevated tastes. She owns a Chicago Symphony sweatshirt and another about Jazz. At Frank's house, she admires the original paintings and artwork alongside Richie. She and Carmy attend performances, visit museums and art galleries. With Syd, Carmy does not have to hide his artistic sensibilities and finds his circle expanding to include people happy to discuss these things with him without mockery.
Carmy loves cuddling with Syd. Soon after they move in together, he upgrades her couch to a larger L-shaped sofa that they can both cuddle up together on. He also insists on a flatscreen TV instead of her laptop so they can watch cooking shows together. They murmur things to each other at these times in short often one-word bursts that would be unintelligible to anyone else (like she'll watch the Nonna kneading bread and say "Rosemary" to which Carmy will respond "with the lardo" because he instantly clocks what she meant).
Syd won't let Carmy cook for her for quite some time after he confesses he has fallen out of love with cooking. She makes non-Italian food and he canonically thinks her food is the best he has ever had. Italian dishes have traumatic associations for him whereas Syd's jollof rice soon becomes a new favorite. He has to grovel for a long time to get her to make her cola braised short ribs with risotto for him which is actually the best thing he ever ate (canon).
He can only fall asleep if she is next to him, pinned to the mattress with his arm and leg over her. He used to struggle to fall asleep and suffer from nightmares. Now he gets a sweet restful sleep and Syd has to squirm out of his grasp each morning because he tries to keep her in bed.
Syd has Carmy's artwork framed and hung above her Tyler Mitchell photos. The drawings he did of her hang in their bedroom. Mikey was the only person Carmy ever gifted his art to before Syd. The way his Xmas present to Mikey of the Bear restaurant was relegated to the Beef's bathroom made Carmy reluctant to give anyone his artwork. Syd's vocal appreciation for his artwork (with no underlying mockery) delights Carmy and makes him blush all over his face and neck.
A few times a month Syd and Carmy go to the movies with Marcus, Tina, Sweeps, Nat, Pete and Richie. Sometimes they watch something at each other's places. Carmy is at first resistant to sacrificing Syd time to Richie but she convinces him that Richie is part of their family lives. Eventually they all come to enjoy arguing with Richie about his hyperfixations on Friedkin, Phillip K. Dick, etc. The Faks, with their undying loyalty that is entirely unearned to Red Flag and disloyalty to Syd as head Chef who has been nothing but good to them, have been fired by Richie when he returns from his Japan trip. Ted was fired first and Neil was eased out of his position to be replaced by people who actually know how to work for a living. Anyone who does not treat Syd well (and even those who disrespect Carmy) are excluded from the tight circle that surrounds Syd and Carmy.
Due to a fat bonus earned from hosting a late night work event for Frank's tech bro buddies at the Bear, the restaurant can close early for the Xmas holidays. This Xmas is spent at Emmanuel's on the big dinner table with members of Syd's family including T.J., and Marcus brings his Dad and his mysterious brother. People share the work of cooking (Marcus on dessert duty, Tina brings side dishes) which means Syd is not overwhelmed the way DD was on Xmas. It is a cherished new memory that overwrites the traumatic associations Carmy had with Xmas post-Fishes. He also asks Emmanuel formally for Syd's hand which makes this memory even sweeter.
Carmy plans to take Syd to Japan and reluctantly concedes to having Richie and Jess, Pete and Nat with baby Sophie and Sweeps joining them. Richie has read so much about Japan in preparation for the trip that he can't shut up about it.