My Mac does "copy on select" - how to disable it?

I had the same issue and by trial-and-error found out, that it only happened while the Unclutter App is running.

The issue was solved for me after I quit Unclutter. Though this solved the issue for me, your behavior might be caused by some other app...

General recommendation

My suggestion is, that you first quit all clipboard-managers (if you have any). If that does not solve the issue, quit one app after the other, even Dropbox, etc.

Also test, if the issue happens inside a specific editor only. For example, if it only happens inside Visual Studio Code, then you might have installed a plugin in Visual Studio Code that enables the behavior.

When the problem is not limited to a single editor, open your "System Settings > Security > Privacy" panel and check, which apps have access to "Accessibility". Try to revoke all Accessibility permissions...