SD-Card slot is NOT under HDMI, neither below physically nor literally underneath the HDMI when the HDMI is unplugged.
It is not visible anywhere on the back, even removing USB drive and Network cable doesn't show a slot, even using a head lamp and a magnifier.
Nor is it on another edge, the bottom, or the top.
I have an older Roku Ultra (probably 2018-ish but it no longer works thus the upgrade) where the SD-Card slow is physically below the HDMI and visible even when the HDMI is plugged into the Roku so I do know the precise location of the SD-Card on that model.
At least 1 earlier message indicates the more recent Ultras have no slot, but channels seem slow loading and sometimes the message suggesting an SDCard will improve loads is displayed.