iPhone has the better reputation for privacy/security, however it is proprietary software and so no one can really tell for sure what kind of data that Apple might collect on you.
It is FireFox that has hidden settings (about:config) adjusted for privacy. It refers to using a js file such as betterfox or arkenfox, or a pre-hardened fork of FireFox (Librewolf on desktop or Mull Browser on Android). You can improve your privacy by using a hardened FireFox, however you may also encounter more issues with website compatibility. It is a trade-off, and is usually only recommended for advanced users.
As Lemmy grows more popular and more people join, eventually more transphobes and other trolls will find a place on the site. If the mods of your instance defederate from instances that host transphobic communities or users, that would help. The instance that this community is hosted on has been doing a good job of that.
In general, you do have to manually download and install updates. However, some apps will automatically check and download their own updates. Also, if the APK is available on a site such as Github or Gitlab, there is an app called Obtainium that you can use. It will notify you when an update is available, and download and install it for you if you choose: github.com/ImranR98/Obtainium