You've probably looked only at the Bazel BUILD files. They are indeed quite declarative (as the syntax is restricted even more).

If you open other Starlark files that have functions (in Bazel, that would be in .bzl files), you should recognize the Python syntax (e.g. `def` statements + space indentation).

Erm what? It's very very obviously based on Python. The docs even explicitly say that. This is the example they give:

  def fizz_buzz(n):
    """Print Fizz Buzz numbers from 1 to n."""
    for i in range(1, n + 1):
      s = ""
      if i % 3 == 0:
        s += "Fizz"
      if i % 5 == 0:
        s += "Buzz"
      print(s if s else i)

  fizz_buzz(20)