In short: Femboy is about presenting as a very feminine man, men doing that can include trans men.

Slightly longer: It’s why there are different categories for sex, gender assigned at birth, gender identity and gender expression. A femboy would identify male and express feminine. The opposite is a tomboy, somebody who identifies as female and expresses in a masculine manner, both are not necessarily linked to the individuals sex and/or gender assigned at birth.

Trans refers to a different gender identity than the gender assigned at birth and doen’t come into play here.