Half Life Alyx is built on Source 2, which was extended from Source 1, which is a heavily modified version of GoldSrc which was based on the Quake 1 engine.

John Carmack said in 2004 that "there are still bits of early Quake code in Half-Life 2" - https://github.com/ESWAT/john-carmack-plan-archive/blob/mast...

https://en.wikipedia.org/wiki/Source_2