I believe this is not hard to detect. Apple should detect this and report such an installer as particularly risky. Chances are the majority of installers working this way actually are malware, legitimate apps like Zoom and WebEx probably are exceptions.
That's extremely unlikely. Malware on macOS isn't prevalent. There is no market for anti-virus vendors on macOS, and Apple have been repeatedly tightening the approval process for macOS software. Gatekeeper only ever gets more aggressive, not less. Meanwhile videocall software is widespread, it's rapidly become a necessity for a large part of the world's population. I wouldn't be surprised if on macOS it's now in sec…
Sounds questionable in all the parts.
Mac: Just click-mount an installation disk image and drag an app icon to the Applicationss folder - isn't this a perfect install UX? If an app installed this way wants to handle some URLs it should declare that in its metadata. No app should be allowed to modify files outside its dedicated directories unless modifying those files is its actual mission.
Linux: just type "sudo apt install app_name" - what can be more handy?
Windows: let every app you install do anything it wants with all the system files, leaving traces after uninstallation is a norm.
The only problems with iOS are it removes a user's right to program his own device freely and demands too much money from 3rd party devs.