Kriegsman 2oom | GL-21 Enjoyer
I thought the weird Doom lore was that he entered the Eternal reality after being banished from his original one; making the Demon Lord the one who modelled himself on Doom Guy
So the way Eternal explains it is that Doomguy from Doom 1 is the same Doomguy as the one in Eternal. The Demon Lord and God are one in the same entity in Doom lore, and God made Doomguy in his image since Doom 1 with the explicit plan of predicting all the events that would happen ahead of time to break him out of his prison built by the Maykrs. Doomguy fights through the events of Doom 1 and Doom 2 (Doom 3 is a reimagining of Doom 1 technically so it isn't part of the lore), locks himself in Hell to prevent the demons from invading again which is where Doom 64 takes place at, then the Night Sentinels recover him, then there is a timeskip to where Doomguy becomes the Doomslayer, the events of Doom Dark Ages happen, then Doom 2016 and then Doom Eternal.