I had to install Windows 7 to support some older capture hardware with drivers and editing software that only worked under Windows 7, no Windows 8 and no Windows 10.
I got a few unfamiliar errors.
The first was I mistakenly used a Windows 7 (no Service Pack) install ISO image and then when I tried to install Windows Updates I had saved manually.. I got one to install and then all subsequent said "Not applicable to your version of Windows)
You have to install with a Windows 7 with SP1 slipstreamed ISO to succeed at Updating it with saved Updates.
So Windows updates meant for Windows 7 with SP1 , barely, or will not work with Windows 7 with no SP1 installed.
Same thing happens if you use a Windows 7 32 bit ISO and not the Windows 7 64 bit ISO, but that's easier to recognize.. I think it might have even a more specific error message hinting at the architecture not being right.
Then once you are installing the Windows Updates there are several critical de-marcations that really should have been Service Packs.. but since the culture had changed they couldn't use old terminology.
01 - 2015 April Servicing Stack Update
02 - 2016 April convenience-rollup (Windows 7 Service Pack 2)
03 - 2016 July and August Update Rollup
04 - 2019 March Servicing Stack Update and SHA 2 update
After all of this.. amazingly.. Online Activation "works"
Until now I had to activate using the phone app.. but apparently its a matter of certificates and algorithms for online communications.
There are other Updates, but for various reasons.. you might want to not install them since they start monitoring or nagging the user about Updating to Windows 10 or later.
You also need to consider using this for "a specific purpose" rather than "general purpose" since it will be less secure than other operating systems and no longer receives security updates.