These are the BigBlueButton NixOS modules.
Since they partly depend on helsinki modules, we provide some of them (and stubs) in the helsinki/ directory.
Since you're probably not using helsinki, you probably need something like:
{
imports = [
"${path/to/bbb4nix}/modules"
"${path/to/bbb4nix}/modules/helsinki"
];lib.nix contains some library functions we share across modules.
Every BigBlueButton component (and also stuff that's not in nixpkgs 20.03) has its own module.
There is also the simple module (simple.nix) which configures the modules for running a BigBlueButton instance on a single host.