Majority of them are just scams. There's no way on earth that you would mine anything worthwhile from a mobile phone. Back in the day when we could CPU mine, sure, you might have earned something. However, today it's all about ASICs (Application Specific Integrated Circuits) - machines that are designed to mine bitcoin. The only issue is, to mine anything decent, they use a LOT of power. And they being, you need more than one. And it's the electricity cost to run them that is the killer.