You may have seen some information online which talks about assigning supplies individually to buildings in the base builder from a supplies bar at the bottom of the screen. This describes how the base builder used to work in the original Zombies, Run! 1 app.
The base builder now in Zombies, Run! (used from version 2 onwards) works differently to the base in the original Season 1 app, in that supplies you pick up are not individually allocated to buildings any more. All the supplies now go into a sort of 'stockpile' and you use them like currency to build your base. Each building will have a supplies and materials cost shown next to it so you know how many of each you'll need to spend to construct that building.
If you want to know more about how materials and supplies work take a look at the What's the difference between Supplies and Materials? page in the Base Builder section.