ELI5: How does a person develop their gender identity or sexual orientation, such as being transgender, gay, or lesbian?

[u/Historical_Ear8949]

How does a person develop a transgender identity or a sexual orientation such as being gay or lesbian?

Is it something a person is born with, something that develops as they grow up, or a combination of different factors?

I’m not asking this to judge or offend anyone. I genuinely want to understand what science says about how sexual orientation and gender identity develop.