Modding

Overall, it's not that difficult to implement, even for AI. Maps often use scripts to enhance AI by accelerating building construction. For the player, it can be done relatively seamlessly through a custom map editor. If you study how functions like GetTownBuildingLevel and UpgradeTownBuilding work, you'll understand how to build a similar system.

Building bonuses – it depends on what is meant. The values of creature stat increases are related to the files of the creatures themselves. The parameters of some special buildings for cities can be found in DefaultStats. The files that define the mechanics of buildings can be found in GameMechanics/TownBuildingSharedStats.