Short answers, pulled from the story.
Most scholars agree that Snorri Sturluson wrote the Heimskringla collection. He lived between 1178 and 1241 as an Icelandic knight, poet, and historian. The first known attribution to him appears only in sixteenth-century Danish translations by Peder Claussøn Friis and Laurents Hanssøn.
The year 1230 marks the approximate time when a massive collection of Old Norse kings' sagas took shape in Iceland. The narrative begins with the Yngling dynasty and continues through centuries until the death of pretender Eystein Meyla in 1177. It covers events from the legendary arrival of Freyr in Scandinavia up to the twelfth century.
Harald Fairhair ruled Norway during the ninth century and marks the start of recorded historical kingship in the text. A central portion focuses on Saint Olaf II whose fifteen-year reign occupies about one-third of the entire work. Another major segment follows Harald Hardrada who died at the Battle of Stamford Bridge in 1066.
Snorri explicitly cites Hryggjarstykki a prose work by Eiríkr Oddsson covering events from 1130 to 1161. He also drew upon Morkinskinna which served as the main reference for years between 1030 and 1177. Skaldic poems composed during the events themselves formed another key pillar of his research alongside oral traditions.
The earliest parchment copy known today is cataloged as Lbs fragm 82 at Iceland's National and University Library. This single vellum leaf dates to around 1260 and belongs to the Saga of St. Olaf. The rest of that original manuscript was destroyed by fire in 1728.