A lot. Running my phone traffic through mitmproxy was a rather sobering experience, especially what leaks on boot before firewall and ad blocker are ready. On Android you can even inspect a lot of encrypted traffic using mitmproxy and the cert in generates, although some apps (like signal) use cert pinning in a way that i haven't managed to get around yet.

Same for Apple. Just add a root authority and you can even decrypt iCloud traffic.