Easiest way to get Windows on my new build

So I have a current build with windows 10 (on external ssd) because my pc doesn't support 11 in its current state. Doing an entire new build, new case, everything. I am now moving to internal ssd. I've read a little bit about cloning but if I didn't want to bother with that, can I just start my PC up with a free copy of windows and then re enter my activation key from my old pc and upgrade to 11? Or is that more complicated?