The more interesting process is the legend printing process, for which there are several:
There is a more reading in the
Keycap printing article on the Deskthority Wiki about regular mass-produced keycaps.
All mass-produced keycaps are injection-moulded: a plastic is melted and injected into a steel mould at high pressure and then cooled.
A full set is usually moulded at once with spouts between all caps - called a "parts tree". The individual caps are then broken off from the parts tree, leaving small marks usually situated at the back of each keycap.
I think Signature Plastics and GMK often cast smaller shots at once, though.
Most mass-produced plastic parts are made the same way. The mould and machine cost a great deal but each individual cast is cheap. That is why small runs of keycaps are more expensive and keyboards made in the tens of thousands are cheap.
I am familiar with injection molding, but not with casting. Would someone please explain what a keycap artisan would actually do in order to produce a keycap design using the casting technique?
You will be able to find lots of articles and Youtube videos if you search for "resin casting" - those should show you the basics. There are two main types of room-temperature resins: (poly)urethane and epoxy, and a large amount of varieties of each, where not every one would withstand being pounded by fingers every day.
Also, to avoid bubbles in in clear casts you would need a compressor and pressure chamber to de-gas the resin before pouring.
