switch-to-prev-buffer is an interactive compiled Lisp function in `window.el'.

(switch-to-prev-buffer &optional WINDOW BURY-OR-KILL)

In WINDOW switch to previous buffer.
WINDOW must be a live window and defaults to the selected one.
Return the buffer switched to, nil if no suitable buffer could be

Optional argument BURY-OR-KILL non-nil means the buffer currently
shown in WINDOW is about to be buried or killed and consequently
shall not be switched to in future invocations of this command.

As a special case, if BURY-OR-KILL equals `append', this means to
move the buffer to the end of WINDOW's previous buffers list so a
future invocation of `switch-to-prev-buffer' less likely switches
to it.