Cool! So yea, the brokerage is a customer-facing firm that provides customer and account servicing plus other end-customer oriented functions. The FCM provides the money handling, other account-based services, trade clearing, and certain back-office/admin services.
FCMs face the brokerage, the exchange venues, and some clients directly.
The brokerage can be an IB (Introducing Broker) or a GIB (Guaranteed Introducing Broker). A GIB means an FCM is guaranteeing the minimum financial requirements for a brokerage. Many times this manifests as an exclusivity with that FCM. Depending on the targeted clientele, this is not a bad sign or arrangement!! OTOH, an IB meets minimum financial requirements and may choose to do business with one or more FCMs... if the brokerage is acceptable to the FCM. Is IB or GIB better? Having immediate access to more than one FCM may expedite account transfers in an FCM default/closure event. But the brokerage will also be strained in many ways in such a situation, so maybe not. The best practice is to have FCM diversification BEFORE such an event occurs.
Brokerages, FCMs, and (applicable) employees, are registered with the NFA (National Futures Association)which itself is regulated by the CFTC. FCMs are also registered with and regulated by the various exchanges where the desired derivative products they wish to deal in are listed/traded. Exchanges set margins among other accounting and trading rules. FCMs set customer account policies, which software/data is acceptable, account requirements, fees, commissions, adjustments to margins, and what have you. Brokerages add their commissions too.
I likely missed a few things, and maybe didn't clarify a few things. If the above answers a few questions, and invokes a few other questions, than I think it's OK. It's been a long time since I thought about the basic infrastructure.
HTH