DOOM: The Dark Ages is placed as a prequel to the DOOM (2016) and DOOM Eternal. It slots in between DOOM 64 and DOOM (2016), where the DOOM Guy escaped from Hell after the ending of D64 and found himself in the realm of the Sentinels/Argenta. He was then kept as a slave there until working his way up into the Argenta's Night Sentinels. Then the war between the Argenta and Hell began, following Hell's invasion.

The Dark Ages will lay out what happened during the war, where DOOM Guy seeks an upper-hand by becoming empowered by the Divinity Machine courtesy of The Seraphim, forging him into the new "DOOM Slayer." The Night Sentinels learn of the Maykrs' decision to side with Hell for the production of Argent Energy and swiftly partner up with DOOM Slayer on a hopeless battle into Hell to stop it. (End of The Dark Ages)

If we recall events from DOOM (2016) We understand that DOOM Slayer survived The Dark Ages' events and since he is now OP, they lock him in a sarcophagus because they can't kill him outright. This is where DOOM (2016) begins on Mars, before the events of DOOM Eternal.

And that's the quick-and-dirty point of where The Dark Ages slots into the DOOM Universe. Hopefully you can put the pieces together if you've played the other games. If not, don't worry as it's not super important and deeply connected to each other. Happy Slaying!

If that makes sense, shoot me an award, if it doesn't, you will be slayed.