Rebooting is simply a case of tapping the Reboot button from the TWRP main screen. You can select to reboot the System (Android), to Power Off, to reboot Recovery, or to boot to the Bootloader. This powers on the device but boots no software, and allows you to access a bricked device via a USB connection to your PC.
In respect to this, what do you do when your phone says Android system recovery?
"Android System Recovery" can be used to boot system, update system from ADB, cache or external hard drive, wipe data, do hard reset/factory reset, wipe cache partition. Usually, the android system recovery stuck problem is resulted from damaged buttons or just hardware issues.
What does it mean to apply update from adb?
Apply update from ADB: The Android Debug Bridge allows you to plug your device into your PC and issue commands from there. It's designed for developers and requires you to install the Android SDK (software development kit).