You install Windows normally, choosing the internal drive (or even another external drive if you want a portable Windows). The external installation drive stays untouched unless you deliberately format it.
You open Command Prompt as Administrator . install windows from external hard drive
Some older computers won’t boot from an external hard drive via USB. And if your external drive is slow (5400 RPM), the initial boot to setup might take a minute or two. But once the Windows installer loads into RAM, speed is fine. You install Windows normally, choosing the internal drive