There is also a Quake sourceport called Mark V that has the Injector built in so you can look for and download maps from within the game itself.
Most new maps will be based on Quakespasm though so if you’re not already using it you might download it just to have it for ease of use:
https://quakewiki.org/wiki/Engines#Unofficial_engine_list
Quakespasm is based on Fitzquake so you can also use that if you like. There is also an update to Quakespasm called Quakespasm Spiked or QSS which some newer maps and mods require to work correctly so just make sure you check those readme files when trying out new maps. It’s not uncommon to have like a dozen different sourceports in your Quake folder (I know I do).
Here is a youtube playlist of recommended single player vanilla maps:
And another for non-vanilla maps:
Here’s a video on how to release a map which also explains how you go about uploading and downloading maps from Quaddicted:
Here are the links to Quaddicted and Quake Injector:
https://www.quaddicted.com/
https://www.quaddicted.com/tools/quake_injector
And this is Func_MSGBoard:
https://www.celephais.net/board/forum.php
This is one of the main hubs of the mapping community and a good place to find threads about new maps to play.
Here is a recent post from the doom thread about SB recommended Quake maps:
Some other big name projects to peep: Quoth, Warpspasm and Arcane Dimensions which tt_zop already mentioned. Most of these projects stray very far away from the vanilla gameplay by adding new monsters and weapons and abilities and things which might not be something you’re into so if you’re looking to keep the experience pure just look for “vanilla Quake maps” like the ones recommended in the playlist I linked up above.
Welcome to Quake!