Price determinations for brokers etc.

Hello,

I am currently trying to get a deeper understanding of how the prices in the market are calculated in a broader sense for Bitcoin especially both also the other leading coins. By the very definition of the decentralised structure of the market, it is not easy to determine from where and how the prices overall are determined.

As I understand it, for a certain exchange, the prices will be determined by the current order book of buy/sell orders on that particular exchange, but how does it work for independent brokers and the like?

Is there a benchmark or standard which is used widely in the private transaction market?

I have looked into both Coindesk and Blockchain.info's API's/price indexes, but I do not see where and from who they are getting their information - and also, is this information only used by private investors (e.g. person-to-person transactions) or is this also the same indexes which are used in brokerage (e.g. large transactions)

Thank you all very much in advance for your insight,