There are a few different elements here to consider, I think. There are some links over here which may be of interest but there are some other things I will add.
Physical traits and intimacy:
First, there’s writing about nonbinary romanceable NPCs. The way you describe them depends a lot on how steamy you’re going, really. If intimate scenes are fade-to-black or relatively light, I wouldn’t think there’s a need to go into a ton of detail, or really treat them particularly differently to a male or female romanceable NPC.
It is helpful to think about characters’ individual relationships with their body or gender regardless of their gender or identity. When I was getting to know my trans and nonbinary characters in Honor Bound, I set time aside to decide details about their bodies, because I knew I wanted to write scenes that described them in more detail, and to write about how they felt about their gender and bodies and how that changed over time; it was so successful that I did the same for the cis characters too and found that it fleshed them out a lot.
This stuff isn’t necessarily something you need the character to say outright, especially if it isn’t a nonbinary character discussing their feelings with a non-cis PC - few people enjoy a Nonbinary 101 Class in the middle of their romance game. But it will inform the way you write about the character, how they feel and present themself, and will make them feel more real to you.
If you’re writing more detailed scenes, it will be more relevant to think about the character’s anatomy; you can make decisions about how you want to describe their bodies based on the character’s personality and relationship with their gender. For example: in Honor Bound there is a point where I describe the chest of a muscular/husky nonbinary transmasculine character who has not had top surgery with a phrase something like “the strong curves of their chest” - because of what I know about this character, I know it would feel odd to say “breasts” there. My goal with the description I chose is to show how the character thinks of themself and shape how the PC/reader sees them also. Ultimately, though, nonbinary characters have bodies and will enjoy them in a variety of ways depending on their preferences - which is where figuring out those preferences for all characters will help.
If you are writing a nonbinary PC - again there is not a necessity to go into huge amounts of anatomical detail, even if there are onscreen explicit scenes. I won’t go into a ton of detail about intimate scenes as we’re not in the adult reader section and it may not be helpful, but it is possible to write a very steamy scene without exactly specifying the PC’s physical setup. Including opportunities for a PC to specify their anatomy is also possible if you’re wanting to go into more detail; some games (The Midnight Saga series comes to mind) also include asking the PC to specify the exact words they prefer for their anatomy.
I would strongly recommend Heart of Battle for excellent steamy scenes involving nonbinary romanceable characters and/or nonbinary PCs.
This is an interesting one, and again it’s applicable for a variety of characters, not just nonbinary ones. Although there are cis women who are very feminine and cis men who are very masculine, there are so many ways of expressing this as well as any number of nuances and subversions that can be done.
I think that it is worth thinking in detail what the character/MC enjoys in their romance life, rather than aspects that necessarily “match” expectations that come with their gender: cis women may want to sweep their partner off their feet (physically or otherwise) or back their partner against a wall, cis men may want to have their tears gently wiped away or be given a scarf that smells of their partner when it’s cold. Or indeed the other way round - the joy of IF is that it’s possible to lean into or subvert gendered tropes and be equally enjoyable! I would like to see options where characters can sit on each other’s laps or swing them around joyfully regardless of gender if they want to, if it’s physically possible to do such things.
Embracing all this means that romantic moments can be applicable to all in a way that feels non-limiting, and a nonbinary character or MC will get to gloriously dive into whatever romantic tropes they find the most delightful. The added bonus is that cis characters and MCs will get to enjoy tropes they like without limitations too.