Transgender is an umbrella term for many identities, including non-binary, genderqueer, and agender.

The "community" is a social and political alliance of people united by shared experiences of identity and a history of advocating for civil rights. A Map of Gender-Diverse Cultures | Independent Lens - PBS

Being transgender is about who you are (gender identity), while being gay or bisexual is about who you are attracted to (sexual orientation). A trans person can have any sexual orientation.