For apps you don't already own, you can manually install IPAs from your computer:
: Forums like Reddit’s r/LegacyJailbreak often share verified links to "cracked" or decrypted IPAs that can be sideloaded without original Apple ID restrictions.
Because Apple has effectively purged iOS 9 compatible apps from the official store (requiring iOS 10, 11, or later now), your personal "library" is the only way to restore functionality to that old iPod touch or iPad 2 sitting in your drawer.