I have a daily cronjob that runs Clam AV, rkhunter and a few others on my Linux laptop & home server. Never really detected anything but I don't even notice the scan running, contrary to any AV software I used on Windows.
At work I work on a system that processes uploads from partners we work with so we incorporate a virus scanner that scans everything that gets uploaded. This is a stand-alone product that scans a file at a time, it never scans the whole disk or does any real-time protection.
Yes, but I simply use the built-in Windows Defender. I've not bothered installing third-party AVs in years as they do not seem to be more reliable and many came with extra security issues (looking at you Comodo)