How will you design the payment system? What tech stack you will use in that? How will you design the distributed cache? Write the code to convert the given number into format as shown- 100 => 100 1000 => 1,000 10000 => 10,000 100000 => 1,00,000

You can prepare for such theoretical questions, as these people are also not that much smart & most times will be fooling around.

It your choice about how to use your time, either by giving interview here or do something useful

Right and also don't be surprised if some 4-5 years experienced person comes to take your Interview for Architect or Team Leader.

Right these people are fake professionals, just wasting the time of candidates, as they don't have anything useful to contribute in company.