Hi, KidTJ,
The procedure is to export your edited movie from iMovie to your desktop. From there drag it, or any video from your desktop, into the import window of a DVD authoring program, or otherwise follow the import instructions of the app. Then burn your DVD to a standard single layer DVD-R disc, at 4x speed. If the authoring program allows you to save to a disc image, do that and then you can burn your DVD from the disc image by control clicking on its icon and selecting Burn Disc from the dropdown menu. That way you can burn several DVDs without having to repeat the encoding process over and over again.
iDVD is not compatible with Catalina. However, one poster on this forum, Batman15, did some testing and recommends replacement software made by Peng GUIPING and is called DVD Creator and is available in the app store for $14.99. Take a look at this thread for his findings:
https://discussions.apple.com/thread/250714476
-- Rich