Most brokers method is fairly simple in that you open an 'money' account with a deposit (wire-transfer, credit card, etc). You can then see your amount in your user profile page. At this point no MT4 accounts are open yet (using MT4 platform as a current example.) From your user profile page you then chose to open on single 'trading' account on MT4 for Forex and CFD for example. Additionally, you may chose to open a MT4 account for crypto currencies only. You can do both, one or the other. With your newly created account(s) you then make an internal transfer to that trading account(s) that you have selected to use in the markets. So the accounts, per say, are not combined. It is simply that you can make internal transfer from your Forex account into your broker cash wallet and then to the Crypto account, or in another pattern that suits you. Other brokers will operate differently for sure, I am just using examples that I know. I hope that clarifies.