dvdproj file on mac with catalina

You can't. That is an iDVD project file and can only be opened with iDVD on Mojave or earlier systems.

The only iDVD project files that one can recover the source material from are those that have been archived and have Archived tacked on to the end of the file name like this: Combo Slideshow Test Archived.dvdproj. Archived projects copy the source files into the project file package. Otherwise the sourdfiles are only referenced.