Converting idvdproj. file to iMovie or MP4

.dvdproj files can only be opened with iDVD, a 32-bit app discontinued by Apple long ago (2011).

You could attempt to retrieve audio, video & still assets that may be inside a .dvdproj file by control-clicking the file then select Show Package Contents and navigate to Content > Resources > Assets to see if there anything usable there. In my recollection, however, only archived.dvdproj. files contained all the source material.

If you are really on 10.12 you can run iDVD if you can find it. Mojave can run it from an external drive.