The file is encrypted with the passcode and the database can be extracted.
1. It is non-incremental. This means you'll need about as much free space on your phone as your Signal database takes, and it may take many hours to make if your database is large (mine is 18GB). I used to wake up to find my phone had not even fully charged because it had been so busy writing Signal backups.
2. Once you have it on disk, how do you get it away from your phone? Especially after SyncThing disappeared from Play Store (because it was basically a non-Android app behind a thin Android shell that couldn't easily be upgraded to more modern native APIs), there's nothing super-obvious here.
I would have loved a better solution for local backups, but realistically, $2/month for cloud backup is really cheap, and a pragmatic solution.
People here seem to want to answer the question of how to copy data most directly, but only because that's how the problem was phrased. I'm not convinced "users had no way to sync data on their phone" was/is a real problem worth paying for YACSS for in the first place.