eDrive sync is static in its configuration which folders it tracks. You’d need to use 3rd party software to setup arbitrary folders, here are some examples mentioned: Synchronization from murenacloud to phone - #8 by tcecyk
Can’t speak to how telegram saves its media, but I’d think the same applies as to Whatsapp.
Signal will not save its media yet unencrypted outside the App. It’s probably coming (“local plaintext backup”) but not yet.