Answering as a trans masculine person who still has large breasts. Hope you dont mind me adding my experiences. I've had two ultrasounds in the last few years, both at the breast clinic after finding lumps.
The first time I was quite nervous about the appointment because I'd had a bad experience in a breast clinic when I went for my surgery consultation. I also worried about feeling out of place (although I know it's not just women who have breast issues, the leaflet I received and everything in the waiting area didn't reassure me)
After a really weird consultation and breast exam I was sent back to the waiting room then quickly called into another room for the ultrasound. Quite a small room, a chair and computer in one side and the other half was curtained off. I was told to go behind the screen, remove everything from my top half and lie down. I was mostly on my back with my hands behind my head but also had to lean to each side.
I was slightly worried about what they might find but they looked at that area first (it had been circled in red pen) and quickly reassured me it was internal bruising. I still have no idea what from. They went over each part of both breasts, and deep into my armpits, as a precaution. Because I was more relaxed I actually quite enjoyed that. It felt like a massage with the warm gel on the ultrasound and the technican moving the tissue around. Armpits were slightly painful with how hard they pressed and nipples felt a bit awkward because they were rock hard from slightly cold rooms.
The second time I had to go thr actual consultation and exam were fine, but the ultrasound was odd. The curtain was left open while I undressed but then they kept on trying to cover the other breast with a thin bit of paper that didn't do anything. Only one breast was scanned and she claimed she couldn't find anything, despite the doctor at the consultation feeling what I had. You have to wait in the waiting room until the doctor looks at the result of the scan and he sent me for a mammogram. The area did show on that, but it was fine