How am I supposed to update and finish iDVD projects with the new macOS Catalina system!?

You can't. You can either revert back to the last system you ran that allowed iDVD to run, Revert your Mac to a previous macOS version - Apple Support, or start over with a new video DVD authoring application from the App Store. There are many listed there and most have demo versions to let you get a feel of how they work before purchasing.

Warning: none are as intuitive or versatile as iDVD was. It is what it is.