FR955 FTP updated even with no bike power meter

Hello after a bike session outdoors, my FR955 updated my FTP.

how does this work? I thought that I needed extra gear such as a power meter on the bike to get the power data.

More generally since I don’t have any power pedals the daily suggested workouts targets some power zones that I’m never able to fulfill. Why not having some biking suggestions not based on power but on say average speed? Or distance. Is this to make me buy expensive gear? Any suggestions as how to get a cheap power meter compatible  with the Garmin ?

The only way I managed to get  biking power data is via an Erg indoor bike at the gym These are the only times where my vo2max is reevaluated if I cycle more than 20mn.

if someone has been through this before thanks for your help!