How do I access old iDVD project files on a new computer?

Good question. I don't have a facility I can recommend from personal experience but I would suggest searching Yelp, Google Maps, Apple Maps, etc. for “iDVD” and/or “DVD authoring”.

For example (Yelp results):

https://www.yelp.com/search?find_desc=iDVD&find_loc=Los+Angeles%2C+CA

https://www.yelp.com/search?find_desc=DVD+authoring&find_loc=Los+Angeles%2C+CA

Hope that helps!

My observations are as follows....

Whilst HandBrake is used by many to "rip" a home made DVD or its disk image I do not believe that it can handle an iDVD project i.e. .dvdproj file.

If a disc has not been burned or a disk image created from the project, then no encoding takes place so no such MPEG 2 video will be seen in "show package contents"

If a disc is burned ( or disk image created) then one or more MPEG 2 files will be seen in " show package contents " but it is video only with no audio, so not much use.

No original media, called "Assets", will be seen in "show package contents" unless File>Archive Project has been performed whilst the project was linked to that original media.If File> Archive Project has been performed then a file called XXXXXArchive.dvdproj is obtained, and the original media can be seen in "show package contents.

Unfortunately, if the Archive process has not been carried out then I don't think there is any useful output to be had from these .dvdproj files, unless somehow the .dvdproj can be re-linked to the original media that was used to build the project.

Sorry that I could not be more positive.

There are video conversion apps like Handbrake that can do this pretty easily. This thread might prove helpful: How to transfer old family DVD movies, in… - Apple Community