Hey Tee2020,
If I understand this correctly, you're seeing 'Not Secure' message in the URL box for a lot of the websites you visit on your Mac and your iPhone. Is that right? If yes, then I'm sure that I can help you.
When you see a 'Not Secure' message in the URL box from the Safari app, it means that the certificate is expired or illegitimate. The Safari app checks the website's certificate and it gives you this type of warning.
Since iOS 13.4 or later and macOS 10.15.4 or later, Safari will also warn you if the certificate is valid but the version of TLS is not secure. I recommend that you avoid entering in your personal credentials from these websites.
You can find more information about this from the article here: If you see a "Not Secure" warning while browsing the web with Safari
I hope this helps answer your question.
Warm regards.