I'm not sure whether the things I do are good, but some of it(I'm on android):

• Disable background data for apps which don't need it
• Disabling unnecessary permissions for apps
• Disabling/uninstalling unwanted apps. You can try android debloater if you want to go further, but would need to be more careful about disabling some important app.
• Using apps like Firefox more than GChrome. Firefox has uBlock origin and other extensions like dark reader n rss radar.
• Not using many connected apps or being tied to a single ecosystem, so as to avoid everything being collected by one group. But if something simplifies your life very much then this may not be practical.
• Gps n mobile data off when not in use. Actually, I started doing this to save battery, but it's standard practise now. If you're a person who gets online calls a lot, turning off mobile data may not be realistic.

If you want to go further, you can try apps like Netguard, which can be used to block and analyse the internet usage of apps on the phone.

I think one can even root their phone and flash custom roms or make modifications of their own. But I've never tried that.