Playing Fire Red!

To level up weak Pokémon, you have to put them at the top/lead and then swap them out to a stronger one where both Pokémon share the XP. Similarly the EXP. Share item that you’ll get eventually if you know where to look distributes XP to your entire party so if you want to focus train, you box non-essential Pokémon. It’s not like the leveling of recent Generations, especially in Generations 1 and 2 where the grind was real(and I’d be one of them vets with Yellow version).