what app will play ISO files in MacOS 10.15.6 Catalina?

ISO is a file type usually associated with either an ISO-9660-format storage volume image, or (confusingly) also associated with a sector-level copy of a volume of an arbitrary volume format and not using the ISO-9660 format or derivative. These copies are sometimes called “images”.

ISO images aren’t usually “played”, and are more commonly “burned” to removable storage, or booted as a guest in a hypervisor, or some other similar application.

Could you elaborate on what sort of data is expected in the particular ISO file here, and your goal past “playing” an ISO file?

https://en.wikipedia.org/wiki/ISO_image

So the question involves this...

...A disk image presumably ripped from a DVD or maybe Blu-ray...

...Processing a disk image without burning it...

...Seeking app updates or alternatives for unspecified 32-bit apps...

What you have is then seemingly a disk image of a DVD; a ripped DVD. Maybe of a Blu-ray.

If VLC can’t play that directly (Advanced File Open in the Media menu seems a good spot to dig), you may be able to mount the disk image and then ask VLC to play that. This if you don’t burn the image to optical media and play that.

I’d check around with the VLC forum and the documentation posted over at the VideoLAN website...

Apps? The Go64 tool is a useful way to find 32-bit apps, and you’ll then want to look for 64-bit replacements, or 64-bit upgrades, or alternatives. And you can choose to delete the rest.

Foyle’s War is available in the Apple iTunes store. For a few dollars, this whole problem goes away.