When I upgrade to os Catalina, how do I get an iDVD application that will work?

There are a number of video DVD authoring apps at the App Store. Many have demo versions you can try before purchasing. If you want to create a presentation consisting of still photos or photos and videos I've only found two that can: Wondershare DVD Creator and DVD Maker Lite - DVD Creator. Of the two Wondershare has the better GUI and is easier to use.

I think one of Apple's high-end video applications (Final Cut Pro?) may allow this but it is costly. I have never used it. Frankly there was no real replacement for iDVD, at least in terms of authoring. Toast I think will do a bit, and Burn (free) does some basic authoring.

Apple stopped supporting iDVD many years ago which means there won't be a version released that will work in Catalina.