What is the advantage of "companion object" vs static keyword

[sodiaan]

@Tim_van_der_Leeuw.1

I’m saying that using statics is often a bad code-smell, an anti-pattern, in a number of ways. Inability to have overloading, or problems mocking static methods for unit testing - have all bitten the code I’ve been working on in the past.

Those are real-life pragmatic issues.

How to mock companion object and how to overload methods in companion object of a super class companion object?
Can you please show some examples?