This is a difficulty hack of Fire Red that has been made with nuzlocking in mind. This means that you’ll frequently encounter routes that only have Pokémon of the same type or that are all good for an upcoming section of the game. Some Pokémon will still be better than others, but you don’t need lucky encounters. If you are crazy enough to attempt to nuzlocke this then I recommend looking up
the documentation.
Every trainer has a new team with custom moves, most trainers have EVs and many also have held items. All potions have been removed from the AI since those can either be abused or they’re completely broken. Most random trainers are skippable just like they were in the original game, but mandatory trainers have been made more difficult. Pretty much all bosses have a full team and the rival’s team changes constantly as it always should have.
Outside of the trainers, Pokémon, wild encounters, and available items little has been changed. The aim was to create a more difficult version of gen 1 and not a version of gen 1 that has everything. This is why you will only see gen 1 Pokémon, their later gen evolutions, and Alolan forms during the main game. Both the player and the AI get very powerful very quickly.
Once you reach Vermillion City you will have access to most moves thanks to a free move relearner that knows egg moves and a ton of TMs from the poke marts, but the AI has access to the same moves. Many useful hidden abilities have been made available as regular abilities and you get access to ability pills before the first gym so that a Pokémon’s usefulness won’t depend on getting lucky with your ability.
To allow certain trainers to reach their full potential the level curve is very steep compared to a normal game. However, you won’t need to grind endlessly. You get a lot of exp, but if you need to train up new Pokémon or want to skip random trainers then you have access to an infinite amount of rare candies. EV training also becomes very easy once you reach Lavender Town.