TWRP is the recovery on the phone and therefore an important part of the software. If you end up there it was either an accident (e.g. you pressed the volume up button during booting) or something is wrong with the software and the operating system couldn’t load, so the phone booted into recovery instead.
Next time you end up there try to just tap on “reboot” and then “reboot system”. If that gets you back into TWRP again instead of booting the phone normally see if your volume button might be stuck and then try again. If it still doesn’t work try manually reinstalling the OS.
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