To super dumb it down, they’re different operating systems. At face value, seeing a command line interface makes it all look the same to the untrained eye, but not all commands and functions are universal.
DOS was largely tied to Microsoft operating systems, and has largely faded out of use. Windows pre-XP was essentially just a skin running on top of DOS, hence why Windows OS’s like Windows 95 had you run in MS-DOS mode to run games in DOS mode.
Unix is a different operating system, but you’ll almost never run into “pure” Unix. I’m not familiar with the terminology, but there’s different variations of it. The most famous or widely used variation of Unix (too lazy to fact check) is MacOS. Like different variations of Windows, Unix based operating systems all have a similar core underneath, but have different “skins” for the user interface.
Linux is another operating system. It’s a Unix-like operating system where it’s an operating system that has similarities to Unix, but isn’t. Like Unix, you’ll rarely run into “pure” Linux. The variations of Linux are called distributions or distros. Some common distros are Ubuntu, Arch, Fedora, Mint, elementary, Debian, Gentoo. They’re essentially like different skins, and some are built on top of each other. Some of it can be convoluted where you have a distro that’s built off of one distro that’s built off of another one. Linux is usually open source and free to download. There are exceptions to this. Linux is pretty popular with computer nerds, because some distros like Gentoo are really popular with tinkerers. It’s also less locked down than Windows and MacOS where if you know terminal commands (Unix and Linux’s term for the command line), you can really have full control over the operating system.
I can go deeper, as I do IT for a living.