Cards on the board are not yet in play. Once buildings are built or purchased, they will come off the board and either be in front of a player or on the table below the board in the town’s own area. The stacks of buildings are really just building proposals waiting to be brought to life. Your worker disc may only enter buildings which are in play.
You can build one of the building proposals by taking a “build” action. Basically these are on the initial few buildings in the town (not on the board). You “enter” the building by placing your worker disc on it, paying any entrance fee for it, and taking the action.
Instead of building you may “purchase” buildings already built in the town or building proposals on the board (or even boats). This represents either hiring someone to do it or buying the rights. You place these in front of you as though you had built them. Note that purchasing buildings and boats is a free action and doesn’t take up your whole turn.
You can build one of the building proposals by taking a “build” action. Basically these are on the initial few buildings in the town (not on the board). You “enter” the building by placing your worker disc on it, paying any entrance fee for it, and taking the action.
Instead of building you may “purchase” buildings already built in the town or building proposals on the board (or even boats). This represents either hiring someone to do it or buying the rights. You place these in front of you as though you had built them. Note that purchasing buildings and boats is a free action and doesn’t take up your whole turn.