Opening old Quicktime .mov Files

Try VLC.

VLC media player - general, free media player that plays just about anything. https://www.videolan.org/vlc/

Some like IINA.

If I had that kind of file I would use Invisor Lite to look at the contents and see which component it has that is something that Quicktime doesn't like. If necessary you can use a free utility such as Handbrake to transcode the movie to something more recent such as H.265 or H.264, with AAC audio.

VLC will almost certainly play them - it's free web download and there are apps for mac, iOS and iPadOs.

Handbrake will almost certainly convert them to some other format if that's what you want. Also a web download.