First, read about the concept of 'difficulty'. The network aims for one new block every 10 minutes. Hashrate fluctuates as miners go online or offline. If new blocks arrive too fast or slow, difficulty is automatically adjusted by the network to compensate. It's like a threshold, the miners need to find a hash lower than this value to succeed.

Based on the current difficulty, one can estimate how many hashes need to be tried, on average, to find one that passes the threshold. Similar to how you can estimate how many combinations you need to try on a combination lock to find the right one, given the number of digits required (more digits means higher difficulty).

The web sites that show graphs of hashpower show this estimate, over time. Compare the graphs for difficulty and hashpower, they have the same form, because one is derived from the other.

A miner who successfully mines a block might actually know exactly how many hash operations they did to find it, which might be different from the estimate. But I don't think they collaborate and report that for accounting anywhere. Someone correct me if I'm wrong.