What's the difference between a desktop environment and an OS?
What are examples of currently used and popular desktop environments like the one I'm using with Windows 11?
An OS is the Operating System (duh!) of the computer. It handles all the communication between the ports (USB, HDMI, etc), video, storage, memory, screen, mouse--essentially all the hardware that makes up your computer. The OS is what handles the communications and translations of the various languages they "speak" so that one piece of hardware can communicate with other pieces of hardware.
A Desktop Environment is the screen that the user sees and and uses to operate the computer. I *would* say Graphical User Interface (GUI), but some DEs can be nothing more than a console screen with a prompt (not very graphical).
Windows typically has just one DE, and that is what people just use. Linux comes with a choice of DE to better fit one's style of usage. COSMIC is the latest DE, joining old-timers like GNOME, KDE, and XFCE (among others).
As others have stated in more detail, Windows and its desktop environment are monolithic; not independent and separable like the architecture of Linux.
The OS is the "mind" of the PC, being software instructions. The desktop environment is the front yard with access to locations and tools.
What are examples of currently used and popular desktop environments like the one I'm using with Windows 11?
Linux has multitudinous "flavors," and quite a few desktop environments, while Windows 11 is similar to 10, though overall negatively changed, as was 10 from 7, while the change from XP was more pronounced. Which gave birth to tweakers to restore some lost or changed functions.