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.
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.