I'm not sure I fully understand your description, but I think you want to download high-quality content in a 9:16 format, suitable for mobile screens. To do this, simply add "oardefault.jpg" to the end of the URL, and you should receive the 9:16 format. A popular tool, like AAAeNOS's YouTube Shorts Thumbnail downloader, uses the same method.

YouTube Official API Docs: YouTube Data API