The stats you see on StatSheet come from a combination of sources:

• STATS LLC provides current season stats plus some historical stats
  
  
• Various individuals have donated stats they've tracked over a period of years (hat tip to Vic and Jeff especially)
  
  
• The NCAA and various teams and conferences have donated stats.  For example, I've started a project to collect boxscores for every D1 school.  Most of the time these come in paper form and I have to do the grunt work to convert them to digital format.
  
  
• I also compute a lot of stats. Tempo-free stats and referee stats are just two examples of a large number of stats that I dynamically compute based on other information in the StatSheet database (e.g., boxscores)