A20 keeps rebooting randomly

@ssaammss random is always a pain to figure out. I would suggest that you start your A20 in safe mode and see if that resolves this. If so, then the issue could be a recently uploaded App. You do need to uninstall those one after the next until it is resolved. To get your phone into safe mode, use the directions from Samsung

If safe mode does not check your phones settings and see if there is a Software update that is available. Update your software if one is available.

Now you can try a factory reset to see if that will resolve it. Just remember that you need to remove the google password before resetting your phone. You will lose all of your images, contacts SMS etc by doing this. Make sure you have a backup of those.

1. Navigate to Settings. and tap on it.
2. Tap on General management .
3. Tap on Reset .
4. Tap on Factory data reset .
5. Tap on Reset .
6. Tap on Delete all .

from here

From a hardware perspective , this could most certainly be a bad battery as well as a loose flex cable from the charging board. Since iFixit doesn't yet have a guide for this, use the one from here as well as this one. I would even do this before doing a hard reset, unless you have your files all backed up. Then it won't matter :-)

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.