I've never 'got'

That used to be a lot clearer back when there were laws restricting social mobility and even how you could spend your money, depending on your class (check out sumptuary laws if you hadn't heard of those). As the aristocracy gradually lost absolute power, class definitions became more implicit than explicit. Outside of simple stereotypes, middle class has mostly become the bit in between and the least well defined. People can find themselves there just because they no longer seem to fit anywhere else.

Getting some of their income from invested capital, basically. There are lots of people who think they're middle class yet get all their income through working. They're wrong.

Attitude, Environment, Language, Manners, Culture etc.

Think about the word 'class', broadly. Its not money.

'blahdeblah was a class act'
'this guitar is in a different class'

There are poor Lords, Baronesses, Heirs to estates etc.

The same things that define upper and working classes really. IMO.