Is there some software to convert VST3 to AU? Or do you develop both separately?
Someone else said
> Almost all VST plugins have an AU version (like 80%-90% or so, and 99% of the major ones).
Which I noticed as well, I wondered if that required a large time investment to support both or there's some API translation layers available.
Historically, there was an API translation layer that VST2 plugins used (called Symbiosis), but these days the vast majority of plugin devs use a framework like JUCE which has multiple different "client-side" API implementations (of VST2, and VST3, and etc) that all wrap around JUCE's native class hierarchy.
There's a few other frameworks floating around (Dplug for writing in D, a few others in C++), but JUCE is far and away the most common.