All customization, except for defining custom completion commands can be done through the customize interface:
Whether the *Completions* buffer should open and update
automatically. Non-nil means automatically open and update. Otherwise,
operate according to completion-auto-help. By default, t
is used. Note that with completions-detailed (introduced in
Emacs 28) enabled, the *Completions* buffer takes much more
time to display. This is also true for other “annotation providers”
such as the Marginalia package. Consider disabling them or this option
on slow computers.
Range of search for a *Completions* window during completion.
A value of t means consider all windows on all frames,
visible means consider all windows on all visible frames,
0 (the number zero) means consider all windows on all visible
and iconified frames and any other value means consider only the
currently selected frame. By default, 0 is used. In Emacs 29+
this option is unavailable since built-in functions and macros are
used, which don’t respect its value. The built-in functions use the
equivalent of setting this option to 0.