Earth

Earth is the third planet in the Solar System and the homeworld of the human race. It is the primary setting for Doom II and Doom Eternal, and is referenced heavily in Doom 3 and Doom (2016). It also briefly appears in the live-action films Doom and Doom: Annihilation.

Doom/Thy Flesh Consumed
Doom's original ending and follow up Thy Flesh Consumed take place upon Doomguy's return to Earth via a teleporter (at the end of Inferno), where he discovers the remains of his pet rabbit Daisy outside of his home city that is now overrun by demons. He fights his way through his hometown, to stop the initial invasion by Hell, setting up Earth's total invasion in Doom II.

Doom II
Earth is where most of Doom II takes place. Immediately following the events of the previous game, Earth has been invaded by the forces of Hell and a large proportion of the human population are killed. The player, who has traveled back to Earth by drop pod after the Hell invasion of Mars, battles the demon forces in order to allow the survivors to escape the ruined planet. At the end of the game, the invasion is defeated, and the survivors are left to presumably resettle and start rebuilding the planet.

Doom 3
In Doom 3, Earth is never seen in the game itself, but is mentioned multiple times as being the center of human civilization, as well as being the ultimate target of Dr. Malcolm Betruger and the demon forces. It is hinted that the ancient Martians may have fled to Earth after the devastating war with Hell, and that humans may be their descendants. It is also mentioned that at some point in the past, the demons were actually on Earth but were defeated and driven into Hell, hence their desire to return and retake the planet.

Doom II RPG
The middle of Doom II RPG is set on Earth, in the East Coast UAC Facility. The three main characters of the game are teleported there after working their way through the Moon's Lunar Outpost.

Doom (2016)
Earth in the 22nd century is heavily populated and prone to energy shortages. Thus the discovery of Argent Energy on Mars was a huge boon, allowing nearly limitless energy potential for humanity. This technological breakthrough was spearheaded by the UAC, a mega-corporation led by Dr. Samuel Hayden to mine Argent Energy as well as research archaeological sites in the dimensions beyond.

Doom Eternal
Following the events of Doom (2016), Earth is invaded and overrun by the forces of Hell. Over 60% of the planet has been consumed by the demons and the majority of the human population have been killed in the first month of the Hell invasion, following the catastrophic failure of virtually every man-made system. The remnants of humanity have either fled the planet, or reside in massive fortress cities under the jurisdiction of the Armored Response Coalition.

Large portions of Earth's environment and atmosphere are rendered extremely inhospitable due to demonic corruption, as cancerous Hellgrowth infestations accelerated the hostile terraformation of the planet and creating Hell portals for demons to travel to Earth. The damage to Earth does not limit itself to the planet's surface--the moon has been partially destroyed, with portions of it forming a ring of debris around Earth.

The Hell invasion soon ended with the deaths of the Hell Priests and defeat of the Icon of Sin at the hands of the Doom Slayer, causing all demons to retreat from Earth. By the events of The Ancient Gods - Part Two Earth has recovered from the invasion, as the planet's environment has returned to a lushest state, and ruined and empty cities are covered in overgrown vegetation. Humanity was left emboldened by the Slayer's triumph over the forces of evil, as the ARC has led resistance efforts in coordinated assaults to purge the remaining demons from Earth.

Doom (film)
In 2026, a wormhole portal known as the Ark was found below the Nevada desert. This portal lead directly to an ancient city on Mars. By 2046, the Union Aerospace Corporation claimed the Ark and built the Olduvai Research Facility on Mars.

Trivia

 * While Doom Eternal currently only refers to one Earth dimension, some other ID software games such as Quake Arena and Wolfenstein: Youngblood do establish there are multiple alternate Earths. It's not clear if this is the case with the DOOM universe (other than if Quake Champions is taken into account). Doom Eternal's plot does seem to imply that the demons are specific to one particular Earth realm, and it might be the same one seen in every game of the DOOM series. The Doom Slayer himself has only been confirmed to having come from Earth originally, but he has not been confirmed to have originated from an alternate Earth as of yet. Neither has it been confirmed where and how or what place in the timeline the events of Classic DOOM/DOOM 64 games take place, other than it is part of the Slayer's past. In an unused line in DOOM Eternal, Khan Maykr states that she will send the Slayer back to his "homeworld, the Earth-world in the seventh dimension, before the time of the demons. You will live there again as before, with no memory of the suffering you have been through. All your loved ones will be there, waiting to embrace you again, as if you have never left..."