In answer to the OP questions, most/(maybe all?) mirrorless cameras operate at the set aperture, so they are always in "depth of field" preview mode.
for example for Sony A7* FF lineup = Lens Aperture Diaphragm during AF might depend on :
(1) camera model
(2) firmware version - see the discussions recently @ dpreview
(3) lens model : native zoom vs native prime vs adapted
(4) display settings : ON/OFF
(5) AF-S vs AF-C
(5) aperture settings on camera (<= F8, >= F8) and
(6) for AF-C was it the initial focus acquisition or maintaining the acquired focus = so whether the focusing is performed
(A) wide open /
(B) open somewhat from aperture setting /
(C) at aperture setting
shall be always tested ...
so any attempts to generalize like " most/(maybe all?) mirrorless cameras operate at the set aperture " are incorrect