When a 64bit App isn’t really 64bit …

For example, an app’s main executable file might be 64-bit but inside the app bundle might be 32-bit code. For example, DEVONthnk Pro office is 64-bit but inside its bundle is a 32-bit app called OCRPlugin.bundle. Sooo … presumably that will no longer run in Catalina?

My understanding is that there are zero support for 32 bit in Catalina so I imagine it will not run.

Yes, you are right. I have a project here that uses a lot of 3rd party dlls and dylibs to control different devices. When I run the Mac build on Catalina, these that are 32 Bit still will not find the library functions anymore.

This is a problem that some App Wrapper customers have run into when they try to post to the Mac App Store also; in that while the main application might be 64-Bit compatible, the nested executables are not and they run into problems because of it.

I’ve have a couple of apps that are t’other way around, the main bundle is marked as being 32-Bit only, while nested executables are 64-Bit.

Only if you are trying to run that 32-bit code. The IDE still contains 32-bit code and it still runs.