Before we get into the "how," let's quickly establish the "why." Fastboot is a protocol used to modify the flash filesystem via a USB connection from a PC.
: Use a USB cable to connect your phone to a computer that has ADB tools installed. how+to+open+fastboot+mode+on+oppo+a74+best
Are you trying to or flash a specific ROM ? Is your device currently stuck on a specific screen? Before we get into the "how," let's quickly
Thus, the pragmatic best practice is: