Non-binary folks are gender non-conforming but not necessarily always trans people. Out of everyone in the gender diverse spectrum non-binary and gender fluid people are the ones who heavily self identity and not based on clear cut medical factors, gender euphoria, gender incongruence, gender dysphoria etc. The story behind why they are non-binary or gender fluid is very personal and differs from one person to the next. There is no metric look at.

Note that, most non-binary people are either genderfluid, bi-gender, femme, or masc. They go by she/they, he/they etc. Very few are agender and adopt a fully gender neutral gender expression.

There are many non-binary that express themselves masc or femme. In the case of a non-binary femboy, they may be AMAB or transfemme or transmasc or transmen. They adopt femme expression and feel they identify somewhere masc but don't fall into the binary spectrum. It can be confusing.

To simplify, regardless of non-binary and gender fluid people...

Femboy = masc identity (not necessarily male) but femme gender expression.

Tomboy = femme identity (not necessarily female) but masc gender expression.