Find My Background Activity drains 20% of my battery in 5 hours

Restart your phone. But Find My doesn’t know where you are without checking; even if you are home, it only knows you are home when it checks. 5 minutes later you may not be home, but it won’t know whether you are or not unless it checks again.

Find My runs any time someone who has either access to your Apple ID or who you have shared your location with checks your location, so someone you have authorized might be continuously tracking you.

If everybody in your house is allowed to share your location their phones will still check your location, whether they are using the phones or not.

And 20% battery drain overnight is normal. Any cell phone uses energy continuously, as long as it isn’t powered off. This means that if it is not connected to power for 8 hours the battery level will be lower at the end of that time. How much lower depends on what apps you have, how many notifications you have enabled (which will be delivered and processed by apps even if you force close apps-they will relaunch when a notification arrives), and, probably most important, how strong your cellular signal from the network is; your phone maintains a "conversation" with the network so your phone can be found when a call or text arrives, and low signal strength requires much more energy to do this update to the network. Thus, you should be charging your phone overnight so this nighttime usage is powered by the external source rather than the battery. And you should enable automatic nightly backups, to protect against disasters.