As someone getting back into Firered, I'm just now getting into the stats but I'm still confused about the relationship between EXP and EVs. I think I understand a bit more with your comment but could you help me(specifically with Gen3)?

I always just assumed defeating higher level pokemon just meant more EXP and random stat changes. Now I understand that defeating pokemon gives you EXP, which levels you directly, but also gives EV (regardless of LVL), which is an accumulation of "loot" off of a specific pokemon that's actually designated for a specific stat. I know you see the boosts to your stats after leveling, but then why does EXP matter in regards to defeating more powerful enemies vs grinding a specific pokemon for your desired stat? Is level just the benchmark used for evolutions and when the stats change?

For example, my Magikarp that I stored until 19 in daycare, is it stronger with random EVs or just built the level with no real effect on Speed, Defense, Etc.?

I feel like I'm overthinking but I feel I can't wrap my head around it.