I will feel bad if this has an obvious answer, but I really did try to solve it myself for a couple of days.
For example: I have a few collections tagged "pop" (Madonna, Britney, Jon), a few collections tagged "dance" (Madonna, Taylor, Cathy), and a few collections tagged "rock" (Alex, David, Jon). If I search for "pop", then the results are Madonna, Britney, Jon; if I search for "rock", the results are Alex, David, and Jon; if I search for "pop rock", the results are "Madonna, Britney, Jon, Alex, David". I want it so that when searching for "pop rock", the results will only be "Jon"; that is to say, I want the results for "pop rock" to be "pop AND rock" instead of "pop AND/OR rock". (So that a search for "dance pop" would *only* yield "Madonna" as a result.)
I didn't find the place to make that change in search.php, or class.store-builder.php, or ajax.php, and of course I checked the functions, defined, and global.php files.