Sure! The main formats are VST3 (from Steinberg, the most popular format), AU (from Apple, supported by most macOS hosts, and Logic only uses AU), AAX (for Avid Pro Tools) and CLAP (an open source variant, that's the best of all of them but is having trouble getting market share). Then there is also "Standalone", which is when you just run your plugin as a standalone app.

Because of this plethora of standards, most plugin developers (including us, I work at XLN Audio) use the JUCE C++ framework, which provides a uniform interface to all these formats and more. It's available on GitHub under a GPL license if you just want to play with it (it has excellent tutorials and example projects). If you're just curious about development thing, I recommend using Reaper as a host to test in, both because it's essentially free (it's free like WinRAR is free), and it has tons of options for how to run plugins (all in the main process, all in a bridged process, every plugin in a dedicated process, etc.).

Audio plugins are essentially dynamic libraries loaded at runtime, and a common way to run them (used to be universal, but some hosts are changing) is that the dynamic library is just loaded in the main process address space and the host communicates with it by calling functions on it. That means that if a single plugin (out of maybe dozens in a project) crashes, it takes the entire host down. In addition, if you have multiple instances of the same plugin (very common, you might have the same effect on multiple tracks, for instance), all global and thread_local variables are shared between them, which makes global variables a total nightmare, and raises the thread_local problem I mentioned earlier.

Our products use JUCE for the unified interface, but then we have an entirely custom Lua codebase for the GUIs and scripting the products themselves (with lots of connections to the audio engine, which is of course C++). There are very limited languages you can embed this way, because of the requirements I mentioned above. The ones I've looked at which you could possibly do it with is Lua, Python (3.12+), JavaScript and Tcl. I haven't done a lot of testing outside of Lua though, this is just me looking at the embedding APIs. You could also do it with web views, and recent versions of JUCE provides nice ways to do that.

I wouldn't do it with Janet if it uses thread local state this way, but maybe it works. It would definitely work if you spun up dedicated threads for each instance and communicated the events back and forth, but that seems like a bad idea.

Let me know if you have more questions, this is a very weird field of programming that most developers are not exposed to.