To maximize privacy do not use a cellular telephone altogether. This will also improve your health because cellular telephone radiation negatively affects your brain and testicles. However, if you necessarily need to use not just a cellular telephone but a so-called "smartphone" it is best to do as you said and install free software programs (so-called "mobile apps" but i dislike the term strongly) on your device. Keep in mind that the mere act of using a cellular telephone nukes your privacy (and your health) so the gain is small, albeit i suppose worthwile to an extent (better to be a slightly less obiedient goy). You could I suppose keep it at home at all times so your privacy is not compromised but the wireless radiation will still affect your health.
Here are some programs you can install on your telephone which are alternatives to common proprietary telephone programs:
app store: f-droid
email program: fairemail
navigation program: organic maps, osmand
browser: ironfox (do not listen to gos devs who think that you should use their vanadium browser. yes, mobils firefox deriviatives have worse exploit protection but using the right extensions is an insane improvement in privacy for an average browser user)
rss reader: any from f droid really, i use readify
file sharing: syncthing, onionshare
password manager: keepassxc
messaging: simplex chat, conversations
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