I’m not sure if you are mixing up AudioUnits and audio plugins in general here?
There are a number of common formats for audio plugins in general, the most important ones are:
- VST(2): The format that started it all, created by Steinberg, available for all operating systems. Steinberg wants it to disappear, so you are not allowed to build new VST2 plugins unless you signed a contract that allows you to do so a few years ago. Sometimes people use VST as a synonym for audio plugins in general
- VST3: The successor of VST, also created by Steinberg. Available for all operating systems. Probably the most future proof format, supported by nearly all mainstream hosts today
- AudioUnit (AU, AUv3): A format created by Apple and therefore only available on macOS. Most DAWs on macOS can load them, Apples DAWs like Logic, GarageBand and MainStage don’t take anything else than AU.
- AAX: A format created by Avid, available for macOS and Windows. Only Avid hosts can load them, and Pro Tools, one of the most popular DAWs in the professional sector won’t take anything else than AAX.
A JUCE plugin project can be used to create all formats from within the same project and sources – which is one of the main selling points of JUCE. So you basically can just develop your plugin in a format and operating system of your choice and then easily build it for other formats and operating systems later from the same code.