grub2 has a builtin scripting language (very much like bash/bourne), which allows you to dynamically do all sorts of stuff.

I did a deep dive into grub2 years ago, and ended up creating my own sort-of-ventoy-like Swiss army knife, boot anything, grub config.

In some ways, you can treat grub2 as a mini os, with a scripting language.

So ... I'm sure that's how Ventoy does it (but I haven't really checked)