They are not always the lowest bidder. Colonial Pipeline was known for selecting the higher bids sometimes in order to ensure quality of work. They'd generally evaluate risk vs. quality vs. price.

Colonial also kept multiple overlapping vendors for their last SCADA upgrade in order to make sure that no contractor was too overloaded during a "boom time". They'd generally stagger the work between locations (they had to upgrade dozens of stations along the pipeline) and keep track of the performance of everyone they hired and try to keep a steady workflow for everyone over a multi-year period.

It was generally not about the lowest bid.