Marcus and Luca are in Sydcarmy S2 stage of their relationship
The show provides lots of subtext to support this. Richie and Jess are in S1 Sydcarmy stage, where barely touching each other makes sparks fly. As @sublimepeachtriumph has pointed out as well, this relationship gets more narrative priority than it deserves and was added near the end of the show.
Back to Caramel. As Donna goes through Carmy's black notebooks in this episode, several things become very clear based on how the show intercuts them with the service going on.
For example, when Donna gets to the scallops, it shows Syd making a scallop in the kitchen that is served to the FOH/Audience. Carmy tells Syd, "Beautiful, chef."
There's a progression of Carmy drawing seafood and cream sauces which I've already mentioned, which are erotic, but he draws foods that represent female reproduction over and over again or look like sperm/eggs. I will presume, that despite his family picking on him for being sensitive and artistic, and calling him slurs, this is the show telling us Carmy is straight. The Copenhagen book is particularly erotic like this, and Donna will return to it later so that Carmy can mention the boat he stayed on with the invisible kitty.
But these notebooks connect Carmy and Marcus's legacies and stories.
Carmy had a sexual awakening of a kind in Copenhagen, or at least of his desires, because he comes up with The Bear then. But he sublimated his feelings into food. When Marcus is sent there, he also has a sexual awakening but doesn't sublimate it.
Instead, Marcus leans into his artistic sensitivity and curiosity, then wins a big award.
Donna reads the Ever notebook and it shows scenes of Carmy, Chef Terry, and Luca in flashback as well as a photo with Terry in between them. This is Carmy's first time experiencing something that feels more familial at a fine dining place. Pre-Copenhagen. The start of Carmy seeing the possibility as the restaurant as family and having it all in one place, which is a dream he shares with Syd, but not Mikey.
It also introduces the point that Luca enters the story in an overarching sense.
When Donna gets to the New York book, she comes across the legerdemain pages. Marcus has read all these notebooks forward and back, and Carmy gifts them to him in the end.
Right after the legerdemain pages, it shows the banana split dessert inspiration that Marcus will craft with Luca and serve that night. (Syd is also shown in S2 eating a sundae alone, eating the cherry on top, wink wink.)
The banana is obvious symbolism, as are the colors of the ice cream. (Neil will later make a lengthy joke at the table about bananas being shoved up asses just in case it's not clear).
Immediately after this, Luca and Marcus have the following exchange related to this dessert. While they are speaking, Syd is cooking the scallop dish. It's about how they've run out of brioche. A very soft and cushy bread, btw. But it also alludes to their meetcute way back in Honeydew in S2.
Luca: Chef, we are running low on brioche.
Marcus: Yeah. fսck. Um, can we make more?
Luca: We haven't got time.
This is telling us that Marcus and Luca's storyline got cut in half.
Just as Syd presents her scallop dish, Carmy says:
It's beautiful, Chef. Hands!
Indicating, that Syd and Carmy's story also was cut short.
These are the only two interracial relationships on the show.
When they decided to end the story on S5, because, obviously, they weren't going to be allowed to progress the way they wanted with these two romantic storylines.
Meanwhile, Nat and Pete get to have sex at a dumpster as @whenmemorydies pointed out, and Jess and Richie get to fly away to Japan.
Carmy steps in and whips up a brioche, though, so his son Marcus and his lover Luca can serve their dessert and let Luca at least meet Marcus' dad before he goes back to Copenhagen.
Carmy never gets to meet Emmanuel as it stands. (Although, Syd has as conversation with her dad and it includes both talk about wearing protection and twins.)
When Claire is inserted in the end of the show (probably contractually), they make sure Carmy's face can't be seen and that Syd is standing next to him the whole time.