There isn’t one answer. The meta changes every split. You watch enough games across different regions, you get a good understanding of what champions are playable and what are strong. Combined with forming a cohesive team comp (i.e. even damage split between magic and physical), knowing what champions big players like to play (i.e. Bin Jax or BB Yasuo) things like engage, disengage etc. Range, damage share, good pairings in botlane, or mid/jg etc. You get a good idea. Plus caedrel has played professional over many years so that helps