If many people have added the encoders in the same way to the same keyboard (e.g. because there's an optional space on the PCB for it), then it's probably just a configuration option on the keyboard firmware.
If you've made a completely custom keyboard, or hacked an existing PCB to add the encoder, then there is a little more configuration.
Either way, there's then a few lines of code to define the action when turning the encoder knob.
The keyboard is configured to either send keys (e.g. PageUp/PageDown, Up/Down, VolumeUp/VolumeDown, AltTab/AltShiftTab) or to send mouse wheel events. You can have it send something different if you're holding "Fn" or whatever, or anything else you can think of.
If you've made a completely custom keyboard, or hacked an existing PCB to add the encoder, then there is a little more configuration.
Either way, there's then a few lines of code to define the action when turning the encoder knob.
The keyboard is configured to either send keys (e.g. PageUp/PageDown, Up/Down, VolumeUp/VolumeDown, AltTab/AltShiftTab) or to send mouse wheel events. You can have it send something different if you're holding "Fn" or whatever, or anything else you can think of.
There's no software needed on the computer.
https://beta.docs.qmk.fm/features/feature_encoders