“Keycap mount” refers to the method of fitting (mounting) a keycap onto a switch. Keycaps come in all colours and sizes as well as varieties such as illuminated and relegendable. Because of this, keycaps are separate parts that fit onto switches.
There is no single or standard approach. Most keycap use a press fit, being held on solely by friction, while other switches use clip-on keycaps. The switch plunger may offer a post for the keycap to press onto, or it may offer a slot for the keycap to press into. The slot approach became common following DIN standardisation as a means of keeping keyboard height low.
Similar-looking keycap mounts may turn out to be compatible, but often these similarities are deceptive: placing a keycap from one brand onto a seemingly-compatible switch from another brand will see the keycap fall off or prevent attachment at all.
See keycap mounts for more details on the individual mounts and mount compatibility.