there is not a specific place where bids happen like the auction of a painting, house, or car.

i'll just give u an example

say a local fire department wants to build an addition to their building.
they have an architect/engineer draw plans and design the addition. A call for bids is then advertised publicly or offered to specific construction companies than then look over the drawings and provide an estimate (bid) for the work, which is then mailed to the governing body of that fire department. the governing body of the FD will then choose the lowest bid (typically).