Every 10 years, after the Census is taken to update population counts in each US state, the boundaries of congressional and state legislative districts are redrawn, in what’s known as the redistricting process.
In most states, the new maps are drawn by the state legislature and then approved by the governor. So partisan control of each of those three political entities is crucial. If a single party controls all three, that party can usually draw whichever maps they want without giving the opposition party any say. If control is split, a compromise will usually result.
Some states, however, have alternative processes set up, instead delegating the process to a commission. The devil is in the details with these commissions — some are essentially just partisan bodies who remain under the parties’ de facto control, while others are more independent entities (like California’s and Arizona’s).