Game concept
Internal Upgrade
The game also links it as Internal Upgrades.
From the game's encyclopedia
Some Domicile Buildings can be improved with Internal Upgrades.
An Upgrade, while similar to other Domicile Buildings, may only be constructed within a specific building. Every building with access to Upgrades have their own unique set of Upgrades available.
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.
Internal Upgrade is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Some Domicile Buildings can be improved with Internal Upgrades." It is part of domicile_buildings. The game also links it as Internal Upgrades. Patch 1.20.1.2, from game/common/game_concepts/07_ep3_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/07_ep3_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.