I’d guess different setups both of the phones and accounts. For example if one has enabled Google location in settings, and stuff like location sharing isn’t disabled, the phone will be pinged much more often. Same if you have office documents on cloud, synchronisation with phone book and stuff like that.
And yea it makes sense that an account that’s more active is of more interest to ping more regularly. Maybe also for security too. Tho I don’t know if it has to be an actual conscious decision by the designers of the systems or just some AI algorithms doing it.
Don’t see why not. You can download a database of hashes and compare that locally. Granted, those hashes aren’t “free”, but that’s due to the legal status of such material. The principle itself - comparing hashes - can be foss.
Yea people can look into the algorithms to see how they work and circumvent etc., but that’s no different than with… Anything else. If someone is motivated enough to distribute the material, they’ll make their own network. Foss doesn’t make any difference here.