compile displays the output buffer using display-buffer (as do most other commands, fwiw). You can see this in compile.el, which is included with Emacs. There is always a link from the help for a function to the source code. Use C-h f compile RET to show the help for this function, then click the link. compile ends by calling compile-start, which does most ...
Here's the definition of deft (version 20210101.1519 on MELPA)
(defun deft ()
"Switch to *Deft* buffer and load files."
(if (not (eq major-mode 'deft-mode))
So it uses your current window, then switch to a buffer with a name of the value of deft-buffer variable, "\*Deft\...