Important bosses are hand selected which is why Gym Leaders have their same typings but with more insane teams and other characters like Hop have teams that match the newly edited dialogue such as his Original Dragon, Resh, Zek and Kyu across the game. In regards to trainers, their Pokemon are also randomly generated through this method, replacing teams with whatever Pokemon is randomly generated (within limits of what Pokemon SwSh has) and upping their levels and movesets to allow for more challenge. That would've replaced something like Nickit or Wooloo for that area's encounter table and this method is continued for every Pokemon.
For example, there's a Croagunk on Route 1 in Extreme. It's not actual randomness, each encounter table is just edited to give the illusion of being random. A Pokemon Scarlet & Violet (PKMN SV) Modding Tool in the Other/Misc category, submitted by ZadenOwer. A tool that generates a pokemon S/V randomized mod. Any and all wild Pokemon, gifted Pokemon (fossils, starters) and static Pokemon (overworld encounters) are 'randomized' by changing what Pokemon appears where, this presumably being updated with patches (same thing that changes Ball Guy to Great Ball Guy). Pokemon 'Primitive' Randomizer - A Modding Tool for Pokemon Scarlet & Violet.
So essentially Sword Extreme, along with the other two Extreme series that Ttar has done, are pre-made ROM hacks.
