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Is there documentation for what each type in the compiler does?  For example: what a `FirReference` signifies?

There is a very <https://github.com/JetBrains/kotlin/blob/cbe7266835961384da184de9809f4ddef58d0244/docs/fir/fir-basics.md|little doc>, but it doesn't explain much. Moreover, it says the following:

> There are no docs about all possible FirElements yet, but you can explore them in compiler code. Most classes for FIR elements are auto generated and written with all possible members explicitly declared, so they are easy to understand.

Regarding your example: `FirReference` represents the reference from some expression to some declaration. E.g. in the following code
```fun foo() {} // (1)

fun test() {
    foo()
}```
there is a function call `foo()` which
• before resolution references to something called `foo`, which is represented by `FirSimpleNamedReference`
• after resolution, when the compiler understood that the function `(1)` is called, the reference in the `FirFunctionCall` is replaced with `FirResolvedNamedReference`, which contains the symbol of the function `(1)`