You may think that Apple’s DVD Player app disappeared with macOS Mojave in 2018. It hasn’t and can be found in macOS Catalina, but Apple apparently wants you to think it has, even though the software has been upgraded to 64-bit.
The easiest way is to use Spotlight. Click the little Spotlight icon in the upper right corner of your Mac’s screen. Search for DVD Player, and the app’s icon will appear at the top of the Top Hits. Double click on the icon and the app will launch.
Once launched you can open a DVD by going to File > Open DVD Player. Of course, since Apple no longer makes a Mac with a built-in DVD player/slot, you’ll need a product such as the $79.99 long-in-the-tooth Apple USB SuperDrive, or better yet, the OWC Slim, an External 6X Blu-ray & CD/DVD Burner!