1. You update something if there is a reason, not updating for the updating. For me, changing Win7 to 10/11 is no good
Software/OS updates are all done for a reason. To fix software bugs/security vulnerabilities and to add more features. No-one isn't spending time and effort, just to jack up software version. E.g from 1.2 to 1.3, without 0 actual changes.
With OS (and software as well), every update has update log with it, that clearly tells what the update changes within the software.
Win7 is end of life. No updates are made for it, since it's too old and it isn't viable to update it anymore (the kernel of it is outdated). So, Microsoft developed new OS, currently Win10, to replace Win7.
This has gone along since the day Windows 1st launched. It started with Win3.1. Then Came Win95, then Win98 (and also Win98SE). Then Win2000 and WinME. After that, WinXP and it's service packs. WinVista was next in line. After that came Win7. Then Win8 and Win8.1. Now, most are using Win10 while the latest is Win11. Win12 is rumored to release Q2 2025.
Many programs that are running on your Win7 are also outdated, since they won't be updated either.
E.g;
Latest Google Chrome browser version, that supports Win7 is 109.
While the current latest Chrome browser version is 131.
Same with Mozilla Firefox browser. Latest version that supports Win7 is 115.
While the current latest Firefox browser version is 133.
This is so with every software you have. You can not update them since the OS you're using, is end of life.
I know this very well since i also have running WinXP system (that i don't plug to the internet). It is painstaking to find software that still supports WinXP and actually works. While the latest software, which is many ways better than what was available back in WinXP era - won't work on that PC because the OS is outdated and does not support the latest software.
So, there is a reason why to switch OS once what you have becomes outdated. But your reasoning of "i don't want to", is not actually a reason. It is just an excuse.
The day will come, at which point your system will get infected with malware and/or data on it gets corrupted beyond salvation. All thanks to the end of life software (including OS), that is running on your PC. Hence why i sent my condolences.
Some people do not like to take advice of others, instead, want to experience it on their own hide, be it good or bad.
Since you do not want to update OS, your best option would be using GNU/Linux LTS distro. LTS means Long-Term Support and depending on distro, can last up to ~12 years, before OS support ends. And with GNU/Linux distro, you call the shots. Update the OS when you want it, compared to Microsoft who forces you to update the OS.
2/3. Problem is that I see no difference in any setting. Why not just put on the maximums?
Yeah, when you're used to 60 Hz and ~5ms response time, getting higher refresh rate and lower response time monitor may not give any noticeable difference.
Your call if you use the monitor at 60 Hz or 120/180 Hz. Same with response time. Your call which you use it at.
4. Thanks, although theory is not what I need
Practical use case must be done by you, yourself. No-one can do it for you.