Understanding Team Cohesion

I’ve dabbled in online battles. But I had way more fun grinding and building Pokémon, and when it came to putting a team together, I always get stuck. I have like 60-90 battle-ready Pokémon but it feels more like a cargo cult, I do all the work to get the Pokémon battle-ready but I don’t really know how to actually formulate a team that’s worth using.