Welcome to the site!
When you’re studying on WaniKani, you should assume that the compositions are just what the creators felt was easy to remember, or made for an interesting mnemonic story. They aren’t aiming for historical accuracy, if there is some historical composition.
With regard to 万, it basically had that shape as is from when it was originally created thousands of years ago. Apparently one proposed meaning for the original shape is a dancer/musician. So, it’s basically a drawing of that, and not a combination of any other shapes.
The more complex kanji get, the easier it is to see all the combined shapes. The basic ones tend to just be based on drawings of things.
If you’re curious you can look that kind of thing up, but that’s not WaniKani’s focus. They’re just giving you a tool for memorizing the shapes as an English speaker.
Also, note that this character is not actually used to mean 10,000 in traditional Chinese, I’m fairly sure. That character is 萬.
万 was merely used as a replacement for 萬, since 万 is easier to write. And that became more common to do.
So the meaning of 万 didn’t originally have anything to do with numbers. It just absorbed all the meanings 萬 had when people used it that way.
If you’re interested in that kind of thing, you can go down a rabbit hole for every single kanji.