I told you in another thread that you had to have Windows 10 1903 version to allow the Windows 10 RAW extension, to be downloaded https://www.microsoft.com …ge-extension/9nctdw2w1bh8
to work. I also provided instructions on how to see if you had Win 10 1903. Have you done any of that? [Edit] For now do not download or install the previously mentioned extension. Based on some content you've provided elsewhere, you are now running version 1903 and shooting with a Nikon 7200. Go to my reply, #14 below to see some instructions for viewing RAW images from Nikon cameras in Wndows 10.
In checking my own installation of Windows 10 1903 I find I do not need the extension to view my 60D and 80D RAW files as icons and they open without any problems, but the default program that opens them in File Explorer is the WIn 10 default Photos application. That can be changed if desired.
Windows 10 has also changed the name of some shortcuts, folders, etc. from what you may be used to. You likely are not seeing for instance a shortcut called "My Pictures" but with some searching you should find a folder just named "Pictures". Usually the path will be something like C:\Users\SomeUser\Pictures where "SomeUser" is something unique to the specific user. If you expand your "C" drive hierarchy, you will see a folder named "Users". Expand the "Users" folder and you should see things like "All Users", "Default", "Default User", "Public", maybe a few more, and one folder which is your unique folder. Click on that and you will see your specific items, setup related items, etc. This "Users" structure is why several people can use the same PC and each can have security unique to their use. You can also find the folder by using the box in the lower left corner labeled "Type here to search". Simply type in "Pictures" and you should see an entry named "File folder in" and that is likely where your pictures are kept.
You do not want Windows 7. Right now it is a security nightmare and only getting worse as time goes on. It is possible to get the Windows 7 "Windows Photo Viewer" back in Windows 10 if that is what you want. To do that however requires you to move a least one file from a Win 7 machine onto the new PC and then requires a registry hack. With the issues you are having, and your apparent knowledge level (no offense intended), this would not be recommended.