How does FTP History work?

I just signed up for trainerroad and linked my Strava account. It was able to pre-populate my FTP history going back to 2016 from my previous strava rides. I’m wondering what method it uses to determine FTP from these rides?

TR is not determining your FTP, it’s simply pulling data. So I would assume your FTP would be from any previous FTP test you did and/or any time you manually input your FTP into Strava.