Good question. If we abstract the idea a bit, it is similar to how the price of any good is determined. The exchanges all have their own prices that they accept to part with their stacks.

Because they are competing on an open market, and somewhat rational actors (trying to maximize profits but also keep paying their expenses) they will tend hover at the same price as one another.

The reported price just depends on where you look. They are almost never the same price.

You still didn't got it

if an exchange sets it higher than market is, then almost no one is buying; if they set their btc lower, then a lot of people will buy it to sell on price

there's currently millions of bots working on these to farm money of the ppl doing it, and exchanges systems are also doing it to ppl that sell lower than market order

if isnt there a lot of people buying on 100k then it will not reach it