Disclaimer: I don't make keycaps, but I have dabbled in casting things at home before. Anyone feel free to correct me or elaborate. There is also some overlap between "plastic" and "resin" but I'm trying to think of them separately for the purposes of the discussion...
Usually 'plastic' is injection molded which requires specialized equipment and is often used for materials that require heat to mold (e.g. thermoplastics like PBT/ABS/POM).
Resin is a liquid that hardens naturally over time (think laquer or glaze). They often use a catalyst to speed up the hardening process, but do not usually require extra heat or pressure to fill a mold because they harden relatively slowly (10 minutes sometimes w/ a catalyst, compared to thermoplastics which just need to cool) and flow better into small parts of the mold. Resin can be really sturdy. Epoxy/resins are often used for flooring, finishing furniture, etc. because they are easy to apply over a large area and can be very hard. A big reason for using plastics in factories is that it can be molded, cooled, and then popped out of the mold reliably and quickly where resins tend to be a little more touchy and slow but can still produce a really good product.
The biggest thing is just to try different processes and get a setup that works for you and what you want to make. If you watch the guys who make 'Artisan' caps, you'll notice that they have plenty of duds and designs that don't work out as they constantly try new ideas and perfect their process.
