With the later versions of iOS supporting "Files" now, Strongbox points to the file that is stored within the "Sync" (Resilio) location. What this means is that Strongbox is actively reading/writing to that file within the "Sync" location on iOS. Whenever I open the Resilio Sync app it will either push the changes I made on mobile to the server or pull down the file from the server and overwrite the file on iOS, depending on which is the newer file.

Also, I don't use Resilio's proxy servers either, so all of this is 100% running on my own infra. I have Wireguard setup on my home network, so if I am away from home, I can "VPN" in on my phone and then open the Resilio Sync app to sync up remotely.

In Resilio I Share > "Copy to Strongbox". I mark it as Read Only to keep from confusing myself and use a separate "Mobile kdbx" for any passwords I create on Mobile and share that the other direction as needed. (I don't have to use two files, I just find it an easier process that makes it less likely for me to forget to sync one direction or the other and "lose" a password.)

(I miss Windows Phone here in that I could directly share my Resilio file and its sync status with a KDBX app as Resilio was a standard file host. iOS really should standardize something like that instead of forcing a unidirectional Share as the only pattern.)

(ETA: I tried Files which is supposed to do something closer, but it didn't seem to work for me.)

€45 for a 'lifetime' purchase, ouch!