Currently there is not a way to list all builtin/frozen modules.
You need to read through the source to find them.
What if you need to know if the current micropython binary has a module you need?

Python REPL has help("modules") which lists all available modules, which would be useful for micropythons REPL, it could also support builtin_modules and frozen_modules values.

Using vim --version outputs all the compiled in features in vim, micropython could have that too.
Why not have a flag that lists all builtin modules and/or frozen modules?.