Hey dreadedreality,
you need to place the maps downloaded into Documents/Trackmania2020/Maps/Downloaded and then go to Local -> Play Map and open it there.
All Trackmania editions since 2003 use the Gbx file. Since you can install TMNF standalone, it binds it in Windows to the file so you can open it and play it directly. For Trackmania (2020) you need to actually place the file in the folder I described above.