How are the boundaries of congressional districts formed?

Answer 1

They are established by the legislature of each individual state. The boundaries are adjusted every ten years based upon statistics & data from the most recently compiled US Census report.

An often heard term associated with the realignment of these boundaries is "jerrymandering". This term was given to the attempts (often successful) of an incumbent to ensure that when boundaries are redrawn, they are able to move any part of their district where they are weak politically to a new or different district, &/or include any surrounding area where they would expect to be stronger as part of the district they represent.

The result in theory is an easier reelection campaign.