Welcome to Android Central! There are a lot of guides out there about this -- Battery University is probably one of the best places to read up on how to keep batteries healthy:
https://batteryuniversity.com/
Some basic rules of thumb:
1. Avoid deep discharges of the battery on a regular basis -- make it a habit to start charging once the battery reaches about 30-40%.
2. Avoid using the phone in extreme temperatures (especially high heat), and make sure not to store the phone where it can get really hot.
3. If you
really want to prolong the battery's lifespan as much as possible, try not to charge past 80%. This can be a bit more challenging in real world practice, so I usually don't worry about this.
Accubattery is an excellent battery monitor app that can give you a lot of good information, and also has some nice features like a notification when the battery reaches a set level during charging (so that you can avoid charging up to 100%). It can also give you an estimate of the battery's health (on a scale of 0-100%, where anything <85% is a significantly deteriorating battery).
As for file manager apps, the two I use are Files by Google and Total Commander, both of which are free and ad-free.
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