You’d think a network drive would be included. Apple used to also provide instructions for connecting a network drive but they stopped with macOS 15 or 26. Now a drive that is physically connected to a Mac is the only method of using Time Machine that Apple officially recognizes.

edit: to be clear you can still use a network drive, but how well that works is the big question. It will probably depend on the user and the setup. The Macs in my house backup to a share on a Pi 4. They can still restore files from it just fine.

Thunderbolt external SSD, would be the only Apple-ish thing. (Formerly of course, FireWire.)

To be absolutely complete, I could mention that at least one SAS HBA PCIe card exists on the market with support for macOS 27, even. Sometimes bundled with other products...