As another cis dude, I'll follow the traditions of my people and suggest an answer.

People are different, and so they are going to have differing ideas about these categories. Any of these labels are just labels at the end of the day. Now, the terms do have a general area they cover but the precise boundaries they cover will vary. Non binary is that you generally don't feel a deep association with either gender. You might identify with varying sets of aspects of both men and women, but you don't consider yourself either thing. Genderfluid typically means that your identity shifts around. Some days you feel more masculine and others you feel more feminine. Trans is typically means that you have flipped your gender from how you were born.

The actual implications of each of these things aren't set in stone. This is why "passing" gets a bit complicated. All of these things really revolve around how you feel on the inside vs how you feel about yourself. That dysphoria can cause real problems for people, and expressing themselves through being trans/nonbinary/genderfluid helps combat that dysphoria and its associated problems. Whatever helps that person is what is important. So, in the example of a trans person, they may have various reassignment surgeries. Some might only do a top surgery, or no surgery at all. Society may only view some of those people as "passing", but that is society's hangup. None of these things they are doing are for the rest of us, so our perception of them don't really matter.

To give a maybe more relatable example, as a guy with a wiry frame and very little body hair, I don't necessarily "pass" for what many would consider the masculine form. That doesn't mean I'm not a man. Our societal checkboxes we've come up with already have really blurry and ill defined edges. When it comes to trans/nonbinary/genderfluid people, we love to try to sharpen those edges when we really don't need to.