When installing Linux, you first have to partition your hard drive.
You can create a seperate partition for your /home folder in addition to the one you create for the rest of the system.
Then when you install a different distro, you can tell the installer to use your /home partition without changing or formatting it. After installation, you will have the new Linux system and the /home folder from your old one. That way, all user settings and flatpak settings will be the same as before reinstalling.
But if you’re a new Linux user, I don’t know how helpful this is. It’s easier to just copy everything in /home to an external drive, then copy it back after you reinstalled, for the same effect.