Is the global hash rate an aproximation?

I'm unsure where this number of the global hasing power comes from.

Currently the bitcoin network hash rate is 192 million TH/s. Where does this number come from? I assume the mining difficulty plays a big part in this calculation but I don't see how that lets me calculate the total hash rate.

In theory the difficulty could be incredibly high but a miner could guess the correct number on the first try. Is the hash rate still going to be 192m TH/s or would that make it 1 H/s for this block? How would anyone even figure out how many tries it took a miner?