Put one endstop per axis at a repeatable mechanical reference — typically the same corner for both towers so the wire homes square. On a 4-axis machine that is four switches (X and Y on each tower); many builders home X and Y together per side. Mechanical micro-switches are cheap and reliable; optical/hall work too.
Homing drives each axis slowly toward its switch, backs off, and re-touches for precision, then sets that point as the origin. In FluidNC you configure the homing cycle, direction, and pull-off in the YAML. If you use TMC2209 drivers you can even do sensorless homing (StallGuard) and skip the switches, though it takes a little tuning on a light machine.
Homing drives each axis slowly toward its switch, backs off, and re-touches for precision, then sets that point as the origin. In FluidNC you configure the homing cycle, direction, and pull-off in the YAML. If you use TMC2209 drivers you can even do sensorless homing (StallGuard) and skip the switches, though it takes a little tuning on a light machine.