There are different types of tactical vests. Some are pre-outfitted with holsters and pouches.
There are also Modular vests, which allows you to choose what you'd like to wear on your vests.
These modular vests usually employ the MOLLE system (Modular Light-weight Loadbearing Equipment).
This is a type of webbing, in which you can weave MOLLE pouches.
It enables you to change your vest's setup relatively fast and secure.
MOLLE is very common within the airsoft, and almost all pouches and tactical vests utilise this.