You do not need to purchase windows 10/11 to use it.
And maybe if you really want to purchase windows, you may find very cheap keys online.
In France I can find win11 keys for 20-30 cents on Cdiscount. Most of the time, you just need to input the key after installing windows.
I don’t know about dual boot. Maybe windows has to be installed first. Never tried it.
Tho I know that it is possible during the drive choice, in the windows install, to select an empty space, then clic on the create partition, and there creating a sufficiently big enough partition for windows will create 3 partitions : The boot, reserved and windows. Then just select the main windows partition, and it will auto detect the boot and reserved partition.
However that is happening on an empty drive. I do not know what can happen on a drive where there is already an OS.
Windows 11 can be used, however a oobe command needs to be input at install, without Internet, to not have to use an online account. Tho windows may ask later to connect with an online account.
For an alternative, windows may also be used in a VM. There may also be a way to pass through all the main gpu if needed, and switch between Linux / windows. But I didn’t really use it. So I don’t know where it is or what are the steps.
If the Linux os needs to be used, but the gpu also has to be in the vm, there is a way to split it. Tho the last time I checked (4+ months ago) the project was incompatible with amd due to some kernel/driver stuff. I sadly lost the link to it…