For some reason I got into tracking the settlement of different cultures and cities of skyrim.
Nedic cultures:
Ambiguous origin between always being in Tamriel or early waves of atmoran settlers. Possibly both. Known groups include Reachfolk which still remain in the Reach, Men-of-Kreath in Falkreath who lasted as late as 1E 9nth century, and possibly the Rothma in the Rift (they invaded Stonefalls in Morrowind).
Falmer:
Unknown origin. Probably settled and adapted from Aldmer at the same time as Towers such as White-gold were being built in the Merethic era. Settling in the shadow of the Snow-Throat tower. Theoretically existed throughout Skyrim, but true extent unknown. Falkreath is an elven name, Fort Greenwall in the Rift is built on an elven ruin, Solstheim was well settled, they neighboured Sarthaal, and of course - the Chantry of Auri'el where Haafingar, the Reach, and Jehanna border. They pointedly avoided settling in sight of the Skyforge.
Dwemer:
unknown origin. At least two waves. First had to be present at the time of the Falmer genocide. Clan Kragen later migrated from Resdayn after rejecting the First Council in 1E 4th century.
Orcs:
scattered around the Saarthal area after their creation. Continued to establish occasional strongholds throughout history. King Yashnag briefly conquered Falkreath in the 2nd era.
Vampires:
First vampire was created in the Merethic era, in what was probably Shriekwind Bastion in Falkreath. Proliferated across Tamriel by both Molag Bal & Lamae Bal. Vampire clans settled both Skyrim & Blackreach. Includes the Volkihar, Nedic Nighthollow clan below the Reach, and Greymoor keep below what would become Solitude.
Direnni:
skipping ahead because the Nords have such a large section. After the First Empire of the Nords collapsed, the Direnni counter-invaded and gained a strong presence in the Reach for a few decades until King Olaf conquered the Eastern Reach. Thus resulting in the breton similar elven influence of Reachfolk.
Alessians/Colovians:
Likewise skipping ahead. Lumping them together as even if they weren't the same at the time, it was the Alessian invasion that allowed Colovian settlement.
Alessians invaded Falkreath in the 1st era, 5th century. Colovians settled the area and from then on the hold would be interchangably part of the Colovian Estates or Skyrim.
Now the big one.
The Atmorans/Nords:
(Going by city & hold at a time).
Saarthal: first colony. Wiped out by Falmer.
Winterhold: initial landing site of the 500 companions. In the early first era Shalidor used magic to expand the city & the College.
Windhelm: founded by Ysgramor as his home city following the Return.
Bromjunaar: considered to be as old as Windhelm. Capital of dragon cult. Shalidor added the Labyrinth in early 1st era. Declined and was abandoned.
The Rift: explored by some of the 500 companions, who established Fallowstone Hall at the future Shor's Stone.
Whiterun city: founded by some of the 500 companions who built Jorrvaskr at the Skyforge. Other settlers surrounded them. Dragonsreach built by Olaf in 1st era, 5th century.
Falkreath city: founded by Bjarfrud Skjoralmor at an unknown date.
The eastern Reach: first conquered as part of the First Empire of the Nords, lost to the Direnni, then reclaimed by Olaf. Continious unrest from native reachfolk throughout history.
Dawnstar/The Pale: no records. Considered one of the Old Holds.
Morthal/Hjaalmarch: little history after Bromjunaar declined.
Solitude/Haafingar: started as a fishing village in the shadow of the great arch. Though there are signs of nordic ruins beneath Castle Dour & the vampiric Castle Greymoor below in Blackreach.
City supposedly founded while Alessians were influencing the First Empire of the Nords. Castle Dour & walls were blessed under cyrodilic divines, not nordic pantheon.
Solstheim: settled by dragon cult after defeating Falmer. Split from mainland as a result of battle between Miraak & Vahlok. Skaal descended from loyalists of Vahlok.
Dragon Cult: active in merethic era for some time as Skyrim is being settled. Dying out by the first era. Notably, dragon cult sites can be found in all 9 holds & solstheim, even if the Nords themselves shouldn't have spread so far yet.
Note that the Kingdom of Skyrim as a unified concept was only formed in 1E 143 under High King Harald of Ysgramor.
Before that the Nords would have been split under whatever independent Jarls, chiefs, warriors, or dragon priests they followed.
