From the game's encyclopedia
Buildings are improvements that can be constructed in Holdings.
Many types of Buildings can only be constructed in specific Baronies or in certain Terrain.
Each holding has a limited amount of Building slots, so a holding cannot contain all Building types. County Capitals have access to an additional Building slot.
Duchy Buildings and Special Buildings use their own slots.
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.
Building is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Buildings are improvements that can be constructed in Holdings." It covers Duchy Building, Special Building, Great Building, Economic Building, Military Building and 2 more. The game also links it as Buildings. 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.