I have played HL1 and HL 2 and I still have like 5 more in my backlog. I have no idea what HL 2 Episode 1 and 2 are, Black Mesa is just HL1 HD I think, Deathmatch is multiplayer I think, Deathmatch: Source is in my library too whatever that is, Alyx exists but is only VR I think...
Idk I got into half life thinking I had 2 games to play and now I have no idea what these other games in my library are. Going to have to do a deep dive soon.