This is the base repository of the webpage https://spatialreference.org
Its content is automatically refreshed from the PROJ' SRS database, at each PROJ release using pyproj
It contains several catalogs of Spatial Reference Systems: EPSG, ESRI, IAU:2015, IGNF, NKG and OGC.
To generate the webpage in your computer just run scripts/run.sh (it uses docker) and check the output in the folder dist.
Spatialreference.org has a sibling project under the PROJ umbrella: CRS Explorer. It has a different approach displaying all the available CRSs and filtering them.