More from Apple Support:
Apple said:If Final Cut Pro X projects aren't rendered after updating to macOS Catalina
When you first open an existing Final Cut Pro project after updating to macOS Catalina, Final Cut Pro might start rendering again or might not show video thumbnails or audio waveforms. The size of your Final Cut Pro libraries might also be larger after updating.
Apple said:About incompatible media in iMovie for macOS
As part of the transition to 64-bit technology in macOS, you may see an alert in iMovie about media files that won't be compatible with macOS Catalina.
Before you upgrade to macOS Catalina, you can use iMovie to detect and convert all incompatible media files so they'll be compatible with future versions of macOS. After you upgrade to macOS Catalina, the option to convert the incompatible files will no longer be available.
To make sure new media you create is compatible with macOS Catalina, use cameras and media formats supported by iMovie.
In macOS Catalina, you might see an incompatible media message in the viewer when trying to play incompatible media if you haven't converted it before upgrading to macOS Catalina.