Miners, basically any user on the network with sufficient computing power, vie with each other to be the first to verify the new transactions and solve a cryptographic puzzle unique to that block.
We can use it more generally to talk about turning something good into something bad. And actually, we often use it by turning it into the adjective 'tainted'.
However, pay dirt does not have to be a valuable material found in the ground. These days, it can also describe something that produces money or profit. Or it can even be the money or profit itself.
We could also say these days, or simply the word today, because today does not only refer to the date, the day we're living in now, but also these days, or nowadays.