Mostly from railroad land grants in the late 1800’s. Land along routes was checkerboard divided, odd lots stayed feds, even went to railroad companies.
I was originally remembering a time period when the railroad virtually had public domain authority to seize private lands, but the history of the land grants in Nebraska is actually much worse: https://nebraskastudies.org/en/1850-1874/railroads-settlemen...
The company building the railroad was paid not only in cash, but also land (which was suddenly very valuable next to a transcontinental railway).
But to avoid giving them all the prime spots, the checkerboard system was put in and they were given 50%, so the other 50% could be purchased by anyone.
