From the game's encyclopedia
Cities are the wealthiest Holding Type, favoring Taxes over Levies. They are associated with Republics, and holding them as any of the playable Government Forms will always incur a penalty.
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.
City Holding is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Cities are the wealthiest Holding Type, favoring Taxes over Levies." It is part of Holding Type. The game also links it as City, Cities and City Holdings. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.