>>116014663rd Party is a catch-all for non-Hastak Transformers. It mostly means small run unlicensed productions from a bunch of random manufacturers in China. Price and quality are usually higher than your typical official offering but it really depends on the company and the release. Some are painted head to toe, articulated to the moon, ingeniously engineered, or a terribly toleranced piece of shit.
Releases span over a decade back to the 2010s, but the current popular memes are G1 Masterpiece or legends style releases. There are also a sizable amount of add-on kits for official stuff. You'll find all sorts of iterations of Transformers with Beast Wars, Comics, Games, UT, original takes, etc.