For starters, the sources of the 4KXX projects for the original trilogy are from various theatrical release prints, the ones that the general audience saw in the movie theaters back in their day. These prints are merely copies made from the original camera negative, so, the quality on the prints are a few generations lower than the OCN.
The source of the Blu Ray releases for the original trilogy were the original camera negatives that was digitally scanned, but alterations were made on them—DNR, added CGI special effects, etc.
Harmy’s Despecialized edition of the original trilogy used a variety of sources. The primary source was the Blu Ray. The sources for the original-trilogy footage that were not found on the Blu Ray were 16mm prints and upscaled DVD video. Because these said additional sources had lesser resolution than the 1080p Blu Ray elements, the Despecialized edition was released in 720p.
For Team Negative 1’s 4K projects, each frame from the theatrical prints were digitally scanned in 4K resolution. They are released in UHD and 1080p.