From the game's encyclopedia
The College of Cardinals is the body of Cardinals who advise the Pope and elect the next Head of Faith of The Christian Church.
Every Cardinal is both a candidate and an elector for the Papacy.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
College of Cardinals is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The College of Cardinals is the body of Cardinals who advise the Pope and elect the next Head of Faith of The Christian Church." It is part of Cardinal. Patch 1.20.1.2, from game/common/game_concepts/pam_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/pam_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.