How would I run a bootable Ubuntu Server from an external SSD in Windows?

"Is there a way to do this without using virtualization (virtual box/ vm ware)?"

No: All solutions to the setup you describe --connection using a Windows terminal-- require some form of virtualization. (Including WSL, which seems to be some proprietary form of virtualization)

Whichever Operating System boots on the bare metal controls ALL hardware: The monitor, the keyboard, the disks, the network connection, everything. Only one Operating System can control the system at a time. Switching between Operating Systems in control of the hardware requires a reboot.

It is possible for the Operating System to present emulated hardware for a different OS to run upon and control...and that process is the very definition of virtualization.

To run a portable Linux server or Desktop natively instead of virtualized, consider starting with a LiveUSB, which contains a fully-featured "Try Ubuntu" environment.