The building browser is an in-game page that can be opened to find out which buildings have been built in the selected settlement and which buildings can be built later. The abilty to build a building depends on two things, the size of the settlement and which buildings have already been built.
the size of the settlement and upgrading a settlement require two things, population and government buildings. At a certain point in the popultaion growth of a settlement,the population level becomes high enough to expand the settlement. the population requirements are as follows:
large town: 2,000-5,999
minor city: 6,000-11,999
large city: 12,000- 23,999
huge city: 24,000+
In order to upgrade the settlement, you must build government buildings. many of these require several turns to complete. here is an example from the East Roman Empire:
much like the government buildings, other structures such as farms and temples follow the same system of having to build the previous building level and also require certain settlement sizes before given the option of constuction. here is another example from the Eastern Roman Empire.
large stone wall
epic stone wall
In other factions there are other unique buildings such as taverns or silk roads.