Ok so the current timeline is this: Doom 1 > Doom 2 > Doom 64 > Doom The Dark Ages > Doom 2016 > Doom Eternal
After Doom 64 Doomguy decides to stay in Hell so he can just kill demons forever, at some point gets picked up by the forces of Argent D'Nur who are like space knights fighting evil or something, and they take him in and train him because all he wants to do is Rip and Tear. Dark Ages presumably takes place around this period. Then he gets sealed in a sarcophagus by the Demons because they can't kill him, and is busted out for Doom 2016.
Doom 3 isn't canon, or a different guy is the protag.
Yes I am cursed with this knowledge.