Somebody else may have a better answer to your question. But for what I understand, many large airlines now pay 0 commission to OTA and TA. That means OTA/TA does not get any commission for selling the ticket. However, there is bulk deals that can be negotiated between bigger OTA/TA and the airline directly based on sales performance. So the smaller OTA/TA could not get anything out from the deal as their sales are small. This is really a process for big airlines to eliminate small TAs and not well funded OTA start ups. Large OTA like Expedia or TA like flightcentre still get something from selling ticket based on direct contract and volume of sales.

It is really peanuts per ticket for OTA/TA to get. However, OTA/TA benefits from ticket changing fees. As many ticket changes would require passengers to call their OTA/TA instead of dealing with airlines.

Not only that, large OTA/TA needs to sell airline ticket in order to sell more profitable hotel rooms and experiences. Larger sales also could help big OTA/TA to negotiate with credit card companies for a favourable fees.

However, it is unclear to me about the credit card fees. OTAs like Expedia impose a credit card fee like BA and is based on BA's charges. I have no idea who actually get the money.