Gaining 2 seats: Texas
Gaining 1 seat: Colorado, Florida, Montana, North Carolina, Oregon
Losing 1 seat: California, Illinois, Michigan, New York, Ohio, Pennsylvania, West Virginia
The states with changes in seats are responsible for the redistricting. What this means in practice is that in a state losing one seat, one district will disappear and the map gets redrawn. Then you'll typically have a primary or a general election where both candidates are incumbents. If both incumbents are in the same party, they'll face off in the primary for that party, if they're in different parties they'll face off in the general election. Alternatively, one incumbent may choose to retire.
So are the states without changes; no net changes in seats doesn’t mean no internal population movement that would render existing districts no longer sufficiently equal to satisfy equal representation (the exception being states that have no change in seats and exactly one seat.)
In the states that lose a seat, it depends. Sometimes two current members will face off for the new district that most closely resembles their two prior districts. Sometimes one of the members will just decide to retire.
The state legislators of each state will redistrict based on the census numbers (having the same number of seats doesn't mean the district doesn't change). Then the primary / election process starts with candidates declaring their intention to run for one of those seats. It is possible that two sitting Representatives will run for the same seat due to redistricting.
This is where you are going to see the preferences of the majority party and what Representatives they like. A nationally popular Representative might have their district removed and absorbed into two or more adjoining districts. This is often done when the Representative is more trouble to the local party then useful.