Your guess about
They seem to be the ones that have
been opened
is correct.
Here is some ways you can use to switch between files.
^1 popups loaded files
Command + Option + Up to switch between declaration and implementation files.
Command + Shift + D to quick open project related file.
Command + Option + Shift + T to reveal your file in the group list. And then select related one.
"Popup of include files" is also good one to browse files related to the current one.
"View -> Smart Groups -> All Files | Symbols | etc " and then Tab and type first letters. Tab. And choose one you want. Bind some keys for that.