@LoudLemur speaking generally (and not in the context of selfhosting), the best way apps can and should interact with each other is via API.
For example, if you drop a file somewhere in nextcloud data directory, it won't pick it up (and for good reason). Unless the app is built around the concept of picking up files from filesystem, this system (around volumes) doesn't work. This is why while we have made it possible to share files via volumes in apps, we haven't invested much energy on this. Most apps have their filesystem layout (say lychee, jellyfin, castopod) and even though they store files in filesystem, it's really like a database (i.e opaque) and it's best not to move things around.
