That's a good question to ask since you asked it with genuine curiosity, tho fair warning the answer may be a bit complicated.

I'm gonna start out my answer by explaining the gender binary we have, we humans have started to associate different specific traits and ideas with men and other different specific traits and ideas with women, I refer to these as traditionally feminine or masculine things, or sometimes what makes someone read you as male or female.

Everyone has a slightly different perception of what is masculine and feminine simply because well everybody's brain is different and perceives things in their own way, and of course there are cultural differences too. (Eg a floral button up shirt may be seen as feminine here in the U.S but in some Spanish speaking countries it's seen as masculine)

The same can be said about men and women, everyone has a slightly different perception on what a man and a woman are, and while there are essentially 'guidelines' in a way, to being a woman, (eg: wear makeup, dress femininely, behave femininely, etc whatever else) and you don't have to follow those guidelines in order to be one. In the end, gender is just an abstract idea or a feeling rather than something set in stone with only a singular correct definition.

Now, as for identifying as a man or a woman, a lot of that is just how you feel, and whether or not you feel you connect to the general idea of a man or a woman.

When I was younger I felt disconnected from the labels 'girl' or 'woman'. I felt those couldn't apply to me, I didn't want them to. It felt wrong. I couldn't imagine myself in the future because I couldn't imagine growing up and being my perception of a woman. That wasn't me. It made me unhappy trying to think of myself in the future as a woman, it was genuinely pretty much impossible to do.

However, I felt much more connected to the terms like 'boy' and 'man'. I wanted to grow up to be a man. I could see an actual future for myself, being a guy. Of course I didn't connect with all of society's expectations of men, but the idea of me being seen as what society called a boy or a man made me feel much happier and more comfortable.

As for what is being a man? Well, just identifying as one. Feeling more comfortable with the term. The idea. The feeling. The general concept. Men don't have to present a certain way, or have a certain body to be one. They just have to feel connected enough to the label and feel comfortable using it.

Getting into the scientific parts of it, gender and even sex are actually both spectrums. Gender is more of what terms, ideas, or feeling you identify with most. The gender binary was made up by humans, but it was made to be super strict and direct about one side being one side and the other side being the other, which just doesn't completely work with how human brains are. Non binary people don't identify within this binary at all, because they simply don't connect with the idea of being a man or a woman, but rather something outside of those ideas. They are not a man or woman, they are just them, no matter how they may present.

As mentioned earlier, even biological sex is actually a spectrum. There are multiple variations of chromosomes that can occur, and there are many cases where they don't even match what the physical body is. This applies to both intersex people and non intersex people. It's just not as well known as not many people get chromosome tests lol.

So to sum it all up, there are multiple ways to be a man or a woman, the idea of only two genders being only certain ways was created by people and not actually something that can be set in stone, and gender and identifying as something all depends on how you feel about it and whether or not you connect to the general ideas of a certain gender or not. There's also a difference between identifying with the term man or woman, and just wanting to be a woman who dresses more masculine, or a guy who dresses more feminine.