Windows defender is all you need. I keep the free version of malwarebytes on my pc just in case something seems off. It does not start up or run in the background tho.

Linux.

Windows defender and checking files with virustotal.

I don’t use windows anymore but if I’m entirely honest, the best antivirus is knowing what not to do on the internet. I always found that every (especially the free ones) antivirus was more annoying than it was helpful and honestly I don’t trust them.

Not that data theft from proprietary software is that big of a deal if you’re already using windows

In the past, the Windows built-in security software was garbage and you needed a third party tool for safety. I think Microsoft didn’t want to admit (internally) how bad the problem really was by investing in a proper security team.

That’s no longer true. Modern enterprise user demands forced Microsoft to provide actual security for their OS or risk losing major customers. Today you’re better off using the first-party security software than adding any third-party freeware (less complexity, smaller attack surface).

If you want to make your Windows system more secure, then you should read about (and implement) Windows hardening. It’s only worth adding third-party security tools if you’re paying for something enterprise-grade.

Defender + Common sense.