Valve has a long track record of collaborating with and sometimes even hiring very talented modder groups/individuals.

In this instance, the Black Mesa team's work has been so high quality and very much faithful to the original work (HL1) that they were allowed to actually commercially sell their mod, originally built in the HL2 engine, as its own game, sort of making it an ... officially ssnctioned, unofficial remaster/remake of HL1.

In particular... the Black Mesa team essentially completely reworked and expanded all the Xen levels of HL1... these were always seen as kind of rough, rushed, not as well executed as the rest of the levels of the original HL1, at least by many fans...

Black Mesa originally just wasn't even going to do the Xen levels, because they were so wildly different than the others, and when it originally released as just a mod, and also I think the first commercial release... it didn't have the Xen levels.

People generally thought that that version of Black Mesa was pretty good, but janky and rough ariund the edges... Source as an engine was undergoing some radical reworking at about that same time, as it was evolving from the HL2 source engine into the L4D2, Portal 2, etc, Source engine...

... and eventually, after a long time of figuring out exactly what feature set they'd need to portray a truly remastered 'Xen', and reworking the whole game to run on the required Source features and framework... Black Mesa was finally able to do the big, giant, total overhaul, with the Xen levels, in 2020.

I know much of this because I used to occasionally chat with some members of the earlier mod team way back in the day, some of them would experiment with stuff in Garry's Mod and start threads on Facepunch, before Garry nuked the forums.

If you're interested in other fairly high quality, but not as expansive HL2 mods, I suggest Minerva.

Very, very good level design... I think Valve actually hired the guy that made that mod at some point.