Income doesn’t dictate class, it is often a by product though.
working class is unskilled/semi skilled labour on a wage. No college or uni education,
middle class is skilled on a salary, further education.
Upper class is effectively aristocracy.
with these in broad buckets often comes financial differences ie a labourer often earns less than a doctor type of thing.
but there are overlaps. A semi skilled tradespersons can far,out earn a civil servant for example or a junior doctor . Upper class can be skint. Middle class can be business owners, or even the beckhams of this world, who could buy and sell much of the upper class.
so financially it is concentric circles. But income comes into it purely by default. Many working class people are wealthier than middle class, or even upper, and many middle class people have less income than working class. And many have more than upper class.
thats why it is not focused on income.