A long time ago, Ubuntu actually had a interesting way to install Ubuntu on your PC through Windows. It was called "Wubi" if I remember right.

It was definitely... Odd in how it worked. I believe it created a Windows virtual disk image, stored that image on your Windows filesystem, and then added an entry into the Windows Boot Loader to somehow boot into that. On first boot, it was like Windows where it asked you to create an account and then boom - all done.

And if you no longer wanted Ubuntu, you could just literally uninstall it from the Windows "Add or Remove Programs" menu and it'd remove the boot loader entry, and delete the virtual disk image.

Super super new user friendly. Unfortunately I think the reason why it was discontinued was there was an I/O performance cost from running it in a virtual image - and of course just as it sounds, it was a hacky way to do things. And of course, you couldn't get rid of Windows because Ubuntu was living inside it.

Reminds me of how nowadays I believe Asahi Linux for M1 PCs is installed from within macOS - you don't need to create a boot USB and load it at startup.