check app availability on each mirrors

A mirror is just a copy of the main repo. There is no guarantee that each mirror provides a complete copy. mirror-monitor should survey which files are available. This could be done a couple of ways:

connect via rsync protocol to fetch the full file listing
send a HEAD request to files as listed in index-v1.json, e.g. on APK per-app

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information