Open vault from another app / filen / proton drive / etc?

Is this intentional? :c

On the left is a screenshot from Obsidian → Open/Create Vault
And the right one is from other apps. Like Signal → Add File

Why can I not select other app’s locations (like Filen/ProtonDrive etc) for Obsidian?

N.B. I’d honestly pay for Obsidian Sync. But I’m not comfortable with it not being E2EE, since I have a lot of personal notes in my vault.

You can’t because mobile OSes establish a heavy sandbox on each app.
The apps can use a shared space (the Documents or Download folders in your screenshot).
Alternatively, there a framework, mediated by the OS to access other apps data. However, it is complicated (few apps implement it) and comes with a hefty performance hit.

Obsidian Sync is E2EE.

Omg I didn’t know this!!
Ty for pointing out!