I'm not an expert in this, but I'm aware of subtle differences in Windows Networking between Win7 and Win10/11. I recall Win7 needs a specified Workgroup name for the computers to see each other. Happy to stand corrected though. More on workgroups:
https://www.elevenforum.com/t/change-workgroup-in-windows-11.11747/
The other thing is that if there's wifi networking then the PCs need to see the connection as 'private' and not 'public' - otherwise the resource shares aren't visible.
Then you need to turn on file and printer sharing, which is a little different between Windows versions. And then you need to set up individual folder/drive shares and permissions.
So it's probably a collection of little issues, rather than one big issue.
Network printers should be OS agnostic. We have a Canon printer connected at work directly to the wireless router, and it gets shared by Mac and Windows systems on the network.
Postscript. I agree that NAS seems like the best solution for you. It could be as simple as sticking a USB drive into your router (a lot of them have that capability) if all you need to do is share a couple of files. Or you could apply KISS principle and just email / cloud share the occasional file. You can do peer-to-peer as you stated, but get ready for a lot of sysadmin and account management for managing permissions. For which I suspect you are at the bottom of the learning curve right now. And which you'd likely end up just nominating one specific PC as the share folder... turning it into an ad-hoc NAS.
As
@USAFRet said, this is bad practice and would indicate unsafe setup. You'll want to allocate a network-accessible folder for each PC, and lock out the rest of it. One way round it is to use Windows Remote Desktop (RDP) and just virtual/RDP into the system with a user account.