The zip-to-planningregion.xlsx file contains a crosswalk between CT zipcodes (technically 2020 ZCTAs, or zipcode tabulation areas, which are geographic approximations for zipcodes) and planning regions in CT.
There are 289 zipcodes and 9 planning regions in the state, and the crosswalk is a many-to-one relationship, meaning each zipcode points ("belongs") to only planning region.
Because some zipcodes cross planning region boundaries, this crosswalk is approximate.
In QGIS, open zipcode boundaries, calculate their centroids, and perform nearest neighbor spatial join (NNJoin plugin) to assign the nearest planning region.
Planning Region boundaries: https://www.census.gov/cgi-bin/geo/shapefiles/index.php?layergroup=Counties%20%28and%20equivalent%29&year=2022
Zipcode (ZCTA) boundaries: https://www.census.gov/cgi-bin/geo/shapefiles/index.php?year=2022&layergroup=ZIP+Code+Tabulation+Areas
Released under MIT license. Feel free to use for all sorts of projects. We will appreciate if you credited CTData Collaborative.