the information on how they calculate it is here -
https://www.coindesk.com/price/bitcoin-price-index/though it's an old article so it could've changed.
this is a quick tldr -
"Which bitcoin exchanges does the XBP include?
The criteria for inclusion in the XBP are:
USD exchanges must serve an international customer base.
Exchange must provide a bid-offer spread for an immediate sale (offer) and an immediate purchase (bid).
Minimum trade size must be less than 1,500 USD (9,000 CNY) or equivalent.
Daily trading volume must meet minimum acceptable levels as determined by CoinDesk.
Exchange must represent at least 5% of the total 30-day cumulative volume for all of the exchanges included in the XBP.
The stated and/or actual time for a majority of fiat currency and bitcoin transfers (whether deposits or withdrawals) must not exceed two business days."
so it's all live exchanges, and significant exchanges. some price indices include localbitcoins and others. that's relevant but it skews everything else.