The Legislative Redistricting process happens in the state every 10 years. It is a statutory requirement for municipalities to provide the state with ward data. 

Wisconsin's Local Redistricting Information Legislative Services Technology Bureau (LTSB) Resources
In the State of Wisconsin, the Legislative Technology Services Bureau (LTSB) receives and processes the census data required for local redistricting, as well as provides the software and technology to perform local redistricting.

2021 Redistricting Process Delayed

Delayed Census Means Less Time for Local Redistricting (PDF) by Curt Witynski, Deputy Executive Director, League of Wisconsin Municipalities