FastBoot is a special mode designed to control your smartphone using a computer. Simply speaking, it allows you to change various parameters, flash the device and install many add-ons. If you put the phone into Fastboot mode, then when you connect it to the PC, it is not an external storage device but a terminal from which you can send commands.
This model has nothing to do with the operating system, for it starts before it boots. Moreover, it is possible to enter it even if no operating system exists. For example, on a Xiaomi phone, you must turn off the device completely and simultaneously press the power button and the volume down key.
However, the accidental appearance of the Fastboot logo on the screen can indicate serious problems. Usually, it is caused by deleting an important system file, viruses, or incorrect firmware of the device. Try holding the power key for 20 seconds to get out of this mode.