From what height does opensky monitor?

Hello, I am new to this site but have learned a little by watching Monkey Werx on YT

II am Irish but I live in Cotes d'Armor and there are a lot of annoying small planes that take off close to us and then buzz around continuously all day. No one seems to know who they are - or where they take off from - I don't see them showing up on the opensky site. Does the site only monitor planes above a certain height? If so, can anyone suggest how I find out who has a small biplane and where they keep it and take off from? Local authorities plead ignorance - not unusual

If they are really close and you may be able to see the registration printed on the aircraft having a pattern like F-xxxx or G-xxxx. You may use that to look it up in some public databases (no guarantee it is listed there):

opensky-network.org/aircraft-database
www.airframes.org/