You might test out something like Guardian VPN and see just how much stuff your phone is trying to send over the wire without your consent. (Guardian is iOS-only, afaik, but surely there must be Android equivalents.)
Netguard has per app blocking with logging/alerts, it's open source and on fdroid/play store.
People aren't going to like what they see. One that always annoyed me was a google services app pinging a geoip url everytime I connected to wifi.