Trading platform if I may understand correctly, in forex for retail traders is metatrader platform (the most common one).
Each broker will provide their price feed from data center thru platform for their clients to buy and sell. Each times client buy/sell in any pair, they got commission. So depend on how much they pay for data feed center, their clients will have very much or less currency to trades. However all the broker can provide all major pairs, all major crosses pairs, japan pairs and commodities. They only limit them-self in exotic because not all the clients trade those pair. In sort they will only buy data feed for the pair that their clients trade the most.