Week 2 Muddiest Point

Who/What determines what is stored in cache memory?  Is that something that the user has control over or could change if desired, or is it predetermined when you purchase a computer?  And what sort of data gets put there instead of into the main memory?  When you say that cache memory stores “frequently used instructions and data,” does this mean instructions for, say, simply turning on the computer?  Opening a browser?  Retrieving a Word document from a folder?