Study thumbnails, don’t copy them
Thumbnails belong to the creators who made them. Download them to study — what the face is doing, how many words, which colours stand out — then make your own. Reusing someone’s thumbnail can get your video taken down.
The most useful thumbnails to study are the ones on outliers: videos that beat their channel’s usual. Those are the ones where the packaging did extra work.
Why is the HD version missing sometimes?
YouTube only keeps the 1280×720 version when the creator uploaded one that size. Older videos and some Shorts only have the smaller sizes.