Not medicine, but DS was not getting through Skype interviews for competitive internships, even though he was genuinely well qualified.
Finally we hired someone, a friend of a friend but whose speciality was to teach communication skills to doctors and others, as well as being an examiner for drama exams. Four sessions made a big difference. One was on Skype, another on presentations, another on basic interview dos and don'ts, and the fourth on teaching. You don't know what you don't know till someone tells you and even a bit of understanding gives you confidence.
I was not around for most of them, but did overhear her suggest to DS that for Skype, you could practice by talking to a bedroom wall. Both DH and DD, who are both wfh at the moment are becoming frustrated with the limitations of Zoom, including the difficulty of reading body language and the inability to explore potential areas of difference over a cup of coffee.
I am sure there are now loads of tips on the internet. I would explore those. I might also ask round friends and school for people who interview regularly and see if anyone is willing to give a bit of online interview practice. This practice will be able to sort out some of the basics like background, volume and lighting. Both DH and DD ended up doing quite a lot of furniture rearranging. He is in DS' bedroom and wanted to avoid sun on his screen, but also needed to replace some of the gaming posters in favour of more neutral photos. (No bookshelf...though there is delight in team meetings at the occasional animal or toddle who invades, and sympathy for the colleague who has had to return to her parents home so has a background of childhood photos of herself.)