Android recovery mode

Android recovery mode is a mode of Android used for installing updates and wiping data. It consists of a Linux kernel with ramdisk on a separate partition from the main Android system.
Recovery mode can be useful when a phone is stuck in a bootloop or when it has been infected with malware.

Enablement

The way of entering recovery is different for every vendor.
Examples:

Zebra and symbol devices: left scan/action button
Nexus 7: Volume Up + Volume Down + Power
Samsung Galaxy S3: Volume Up + Home + Power
Motorola Droid X: Home + Power
Older Samsung devices
Samsung Galaxy A10s: Volume Up + Power
Features

Features of the recovery mode usually include:

Applying updates using the Android Debug Bridge
Applying updates from the SD card
Hard resetting
Mounting partitions
Running a system test
Custom recovery

The pre-installed recovery mode on Android can be replaced by other software, such as TWRP or OrangeFox. It can include features such as:

Full backup and restore functionality
Applying unsigned update packages
USB mass storage access to SD cards
Full ADB access, with ADB running as root