Advanced phone customization/Before we start

You would have reached this place likely because

1. Your phone is slow and you're looking for ways to speed it up
2. You're low on space
3. You hate the skin or apps provided by the device manufacturer which you cannot remove directly
4. Anything else

An important term which we'll be using throughout this book is stock. It refers to the OS (with its skins, programs and all) which was shipped with your device manufacturer.

Some common myths

1. It's god: It's not. It can indeed do wonder to your phone if done properly. But neither a root nor installing a custom OS can defeat the phone's hardware: if it is underpowered, then it most likely is. You cannot buy a Nokia 1 and expect to perform it to the level of a Galaxy Note 20.
2. It can fix my camera/speaker/xyz: Rooting is software. It has nothing to do with your hardware. If your camera stopped working, then most likely it has failed and you should take it to a repair shop.
3. Everything'll work: Not necessarily. The quality of the ROM you get is dependent on many factors. Often there are some phone features which are too complex to include in a ROM and may not be included or have issues. A common example is Dual-Sim support; many ROMs have issues or work only with one sim.

The warnings

1. While the risk to your phone is minimal if done properly, as with anything else of this nature, there is a small risk of your phone getting bricked to a state where it's permanently broken. You're doing this purely on your own risk.
2. Your warranty on your phone could be voided, depending on the manufacturer (see special note on Samsung phones below).
3. You'll lose your apps and data during this process; make a backup of them.

For Samsung phones

Most new Samsung phones (from the Galaxy S5-S6 series onwards) use a QFuse which is a hardware feature tied to Samsung's Knox security system. If an attempt to install a custom ROM is detected, it will succeed, but the QFuse (often referred to as the Knox switch) will trip. The below screenshot shows a Note 3 with the Knox switched tripped (notice that that Warranty Void hexadecimal is 0x1 and not 0x0).

This has warranty and service implications for you as it means that you will be unable to obtain in-service warranty as they will notice that you have tampered with the phone. As it is a hardware switch, it cannot be reversed once tripped, even if you later reinstall the stock OS. You may also be unable to access Knox features or use Samsung Pay.

Hence, it is advised that you root/install the custom OS on your Samsung phone after the warranty expires (which is usually for one or two years).