I get market forces, supply and demand etc, but I'm thinking more at a practical level here.

Correct my n00b understanding if it is wrong, but there is a single distributed ledger (Blockchain) which is the Bitcoin implementation. There are nodes which maintain this network and miners which process the transactions. Then there are exchanges which allow you and I to purchase Bitcoin mined on the platform from, I assume, the miners. So at a practical level is is a case that:

• The exchanges purchase a bulk lots of Bitcoin that they can sell on there exchange, or do they have a list of traders/wallets willing to sell that they transact with when I click the buy button?
• Do the miners somehow set a 'reserve' price at which they are willing to sell, or is the price of one BitCoin defined somewhere else by someone else?
• How do the various exchanges determine the price? Whats to stop one exchange selling at $5000 and another for $3000? How do they synchronise their pricing and why do they differ slightly?

Thanks, and merry Christmas to all