While Bethesda Game Studios may not have developed every Fallout game since they took ownership of the franchise, they have no doubt had a profound impact on the direction of the in-universe lore of the series.
Fallout 3 and Fallout 4 both sought to take the series into new directions (whether that be for good or ill), and one of the areas this is most obvious is all the new factions like the Minutemen, characters like Three Dog, and tons of background information that they've sprinkled into their games. Here are just some of the additions Bethesda has made to Fallout's lore over the years.
8 The East Coast Enclave
While Fallout 2 ends with the destruction of the Enclave on the Californian coast, Fallout 3 establishes that many escaped the destruction of the Poseidon Oil Rig. They would eventually take control of Raven Rock, a military installation in the mountains outside Washington D.C. Here they would be led by Colonel Augustus Autumn.
This branch of the Enclave would also have its own president in the form of John Henry Eden, although he is merely a robot who functions as the front-man of the Enclave. It exists for propaganda purposes, delivering radio addresses via eyebots across the wasteland in order to garner support from the local survivors.
7 Galaxy News Radio & Three Dog
While Galaxy News Network was shown in the original Fallout intro, it wasn't until Fallout 3 that players would learn about their radio broadcast division. Three Dog would re-purpose Galaxy News Radio's old tech to deliver his own radio broadcasts across the Capital Wasteland after the war in order to galvanize support for the "good fight" in his words. This takes the form of public service announcements, music, news reports, and radio plays.
Three Dog is an avid supporter of the Brotherhood of Steel and their mission in the Capital Wasteland. This somewhat mirrors the old Galaxy News Network's own support for the U.S. government and its questionable actions just before the Great War.
6 Synths & The Institute
Both Synths and the Institute were first mentioned in Fallout 3's "The Replicated Man" questline. Fallout 4, however, brought both back in a huge way, making them a central focus of the game's main story. The Institute is essentially a continuation of Boston's pre-war scientific community gathered under the Commonwealth Institute of Technology.
Here they conduct a wide variety of experiments of questionable ethics and attempt to influence events in the wasteland above. Whether it's introducing super mutants into the surrounding areas or using the synths they create to replace key members of the Commonwealth, the Institute is clearly playing its own game in the post-war world.
5 The Dunwich Cult
Fallout 3 was the first game in the series to introduce Lovecraftian horror into the lore. This takes the form of the Dunwich cult, which is a mysterious pre-war religious organization that worships a seemingly malevolent entity known as Ug-Qualtoth. References to them can be found in the Dunwich building in Fallout 3 (as well as its DLC, Point Lookout) and Dunwich Borers in Fallout 4.
Not much is known about the cult's activities, although they seem to conduct sermons to honor their god and even engage in human sacrifice in its name.
4 The East Coast Of Former America
Before Bethesda took over the franchise, Fallout was mostly set on the western side of the former United States. This changed with Fallout 3 and 4, both of which were set on the East Coast of the U.S. in Washington D.C and Boston, respectively. The Capital Wasteland refers to the former capital of America and is a heavily irradiated hellscape populated with all manner of mutant monsters and even the psychopathic remnants of the U.S. Government.
The Commonwealth, on the other hand, is not quite as desolate, though it has its own problems. Factions such as the Minutemen, the Railroad, Brotherhood of Steel and the Institute have all taken root in the area, with various settlements propped up in between.
3 Children Of Atom
Much like the Hubologists found in San Francisco, the Children of Atom are a post-war religious group that worships iconography related to the atom, i.e. nuclear power, radiation, atomic bombs, and the like. They can be found in the Capital Wasteland, Commonwealth, and even Far Harbor, where they attempt to spread the word of "Atom" and evangelize his power.
When first encountered they seem to be a peaceful organization, though by the time of Fallout 4, they have split into multiple sub-factions, some of which are more dangerous than others.
2 The Pitt
Pittsburgh is just one of the many new locations brought into the series by Bethesda after they assumed ownership. First seen in Fallout 3's DLC, The Pitt, the once thriving and industrious city is reduced to a skeletal husk of its former self propped up by slave labor.
The city is ruled with an iron fist by former Brotherhood of Steel soldier, Ishmael Ashur, who maintains that one day the city will be free from the sickness and mutated horrors that plague it - just not any time soon. The setting also appears in Fallout 76 as part of the Expeditions: The Pitt update.
1 Aliens
Aliens have always existed in some form in the Fallout games, but it wasn't until Fallout 3 that players got a detailed look at who and what these aliens are with the Mothership Zeta DLC. In it, the Lone Wanderer is abducted by aliens and along with some other abductees is tasked with liberating themselves from their extraterrestrial captors.
In later games, Bethesda would expand on the alien presence in the Fallout world with things like the Cabot family, whose insanity and unnaturally long lifespan is a direct result of contact with alien beings, and the Interloper, a mysterious Lovecraftian creature that can be found in the Lucky Hole Mine.