what year is it now?

let's start with these bits.

what do you think constitutes being a certain gender to a trans person? ... Is it unique to every trans person?

you've answered your first question with your last-- yes, I'd say it is unique. we can think of it like art. if you have a hundred people watch the same movie and ask them their experience of it, you'll get a hundred different answers. it's like that, only we're not even all watching the same movie to begin with.

I have read trans people describing it as an innate feeling and knowledge of being trans.

blessed are those who know from a young age or without question. but in my completely anecdotal perspective, they are the lucky few. the mode is adolescent despair, mid and early life crises, and misery oscillating against endless doubts as to if one is really 'trans enough' to warrant doing anything about it.

but i want to focus on this bit:

Gender norms and roles are largely made up by society, and differ heavily across them. When people transition, are they reinforcing gender stereotypes?

i can see why you would ask that, and on a certain perspective, yes. most basic example: feminine "boy" transitions to be feminine woman, ergo the exceptions conform to the rules and the two categories each take a step to homogeneity.

counterpoint A: to what extent does a person just living their genuine life reinforce a stereotype at all? i am reminded here of the discussion about climate impact-- yeah a person can go vegetarian and practice zero waste and take the bus and etcetcetc but the overall climate impact of that is going be a drop in the bucket compared to systemic changes or corporate giants going greener. i posit that 'reinforcing gender stereotypes' is a concern not for people living their own lives, but for media, representation, or those who would police and belittle others for their habits or presentation. the more true to ourselves the rest of us can be, the better. if doing so creates trends, so be it.

but let's set that aside and take 'reinforcing gender stereotypes' in this context as both a given and something that Ought to be Avoided. after all, the stereotype itself is an obstacle to self-truth. are we reinforcing these stereotypes? i still say no.

counterpoint B: nonbinary trans people, of course. but even then, being nonbinary is not necessarily binary. there's a lot of she/theys out there, for instance, you know?

turns out when you open the door to not being one's assigned gender, a lot of people are eager to jump ship, and not necessarily to The Other designated category. if gender is not boxes, is it spectrum? grid? performance art? all models are wrong, and this one stopped being useful. it having been examined for the shadows on the wall it is, new art may be created incorporating those shadows, but that doesn't mean that watching needs be an exercise in ignorance.

but that gets a little too abstract for discussion. so let's reel it back in and focus just on binary trans people. do binary transpeople reinforce their chosen gender's stereotypes?

...definitely sometimes! i think of the concept of babytrans here. i've heard it described like this: when a lot of girls first transition, they want the longest hair and the shortest skirt. but this is reasonable. to use a physical analogy, if you have a bunch of leftward momentum and you suddenly realize you actually want to be going right, you must apply a lot of force to course-correct. for transfems, hyperfemininity is that force. idk what boys do or like tho.

counterpoint C: ...but what about after that? again, speaking fully in-binary, my personal experience and observation is that once someone has been their own correct gender for long enough, years, decades, whatever, and has finally become comfortable with themselves in their chosen gender, that doesn't always mean they want to adhere to it 100%. as with enbies, the door has already been opened. there's nothing to stop one from crossing back through it now and again. in fact, just recently i was observing a group of transfems trading tips for how best to dress butch/masc while still visibly being women.