It has been my experience while fixing family members computers that unless you uninstall the third-party antivirus software Windows Defender doesn't kick in.

It has been my experience while fixing family members computers that unless you uninstall the third-party antivirus software Windows Defender doesn't kick in. If I recall correctly there was an instance where I tried to switch from third-party to Defender (without uninstalling 3rd party) it would not let me stating that AV was managed by group policy (? I forget the exact phrasing) and I had to uninstall the other antivirus software first.