

I use rsync for that.
As does syncthing under the hood. The issue is with backing up an open database and getting an inconsistent state, but KeepassXC keeps its database closed except on update. I also tick the backup old before save setting in KeepassXC (the aforementioned ‘and it’s backup’) and use a versioning backup of the sync directory on the desktop with 3-2-1, so I am sanguine.
I’m guessing OP means code you use rather than code you write, in other words auditing. Likely very few of us do that with any thoroughness. IIRC proton does have some independent auditing.