My understanding has always been that Docker Engine was only available directly on Linux. If you are running another operating system then you will need to run Docker Desktop (which, in turn, runs a Docker Engine instance in a VM).
This comment kind of makes it sound like maybe you can run Docker Engine directly on these operating systems (MacOS, Windows, etc.), is that the case?
I wanted to offer that the (Rancher Desktop, lima, colima, etc) products also launch a virtual machine and install docker on it, so one doesn't need Docker Desktop to do that. My experience has been that the choice of "frontend" to manage the VM and its software largely comes down to one's comfort level with the CLI, and/or how much customization one wishes over that experience
Quick note that on Windows you don't need docker desktop. It's convenient, but regular docker can be run in WSL2 (which is the same VM that docker desktop uses).
Docker Engine uses a feature of the Linux kernel called namespaces. Alternate OSes require a Linux VM. As another commenter mentioned, apps like Orbstack, Podman Desktop, and Docker Desktop provide a facility to create such a VM.