What exactly is executed asynchronously?

[mechan1sm]

I'm relatively new to neovim and lazy.nvim is my first plugin manager.
I saw "💪 Async execution for improved performance" in the list of features, and now I'm curious what exactly is asynchronous in lazy.nvim and what is not.
I'm sure about plugin downloads, they should be asynchronous.

But I wanted to know about the following things:

1. Are init() functions executed asynchronously?
   What about opts(), config()?

2. Are plugins loaded asynchronously (especially ones at startup)?
   If not, is it even possible to load them in parallel?

3. Does lazy.nvim do asynchronous execution in a single thread, or maybe there is a thread pool?
   What does concurrency(number limit the maximum amount of concurrent tasks) mean in ⚙️ Configuration? Is it the thread pool size? (Can thread pool size be modified?)

4. What lower level library or API does lazy.nvim use for the async stuff? libluv? vim.schedule()?

The more detailed is the answer, the better. Thanks in advance.