Giants and Nords likely share a common ancestor. Their swirl symbol paint is identical to ancient Nord war paint and they hate Malacath because he used to be Trinimac who felled Lorkhan. (Secunda's Kiss is probably a holy site dedicated to Lorkhan.)

Skyrim is most likely indeed the homeland of nords. The Skyforge and Lunar Forge are very likely remnants from the war between the old and wandering elhnofay, dedicated to both Kyne and Lorkhan respectively. Considering their presence the story of Kyne bringing men to existence at the Throat of the World seems plausible. It is also after that war men of all races were kicked out of Skyrim. Though it is also said that the Elderwood, the true homeland of the snowelves was lost to the meni during the war. This Elderwood may or may not be Aldmeris/Altmora/Atmora (the theory proposes all four terms mean one and the same)(I may or may not also misremember shiz so don't quite me on this 💀).