Starting an RIA - Portfolio Management Options

Hi - hopefully I've come to the right place to ask this.

I'm gearing up to register an RIA and I have been trying to figure out how, as a new advisor with no AUM, I can perform portfolio management services for my clients. I don’t qualify to open a custodian account with any of the larger brokerages and I don’t really want to have that kind of control over clients’ accounts anyway (to avoid the appearance of having custody myself). I've researched other advisors and found that a lot of them require that clients open an account with the brokerage of their choice and give the advisor access. What isn’t clicking for me is how advisors get access. I understand that a LPOA can be used to give advisors access to trade in client's accounts, but when I called Schwab to ask about this, I was told that the LPOA is not meant to give advisors access to retail accounts and that advisors should only make trades from institutional accounts. In that case (1) how else can advisors access a client's retail account and (2) even if an advisor has an institutional account at that custodian, the client's retail account would be separate, so... I've seen plenty of advisors that have no stipulation as to which broker a client uses - should I assume that they either have institutional accounts with every custodian or are willing to open institutional accounts as needed to assist respective clients. And, again if that is the case, why would the customer be required to open a retail account?? Also, how are these advisors getting paid when they don’t have a relationship with the custodian? I assume that a customer can request a withdrawal to pay the advisor fee, but I get the impression that advisors are initiating these drafts themselves... I have searched and searched and searched and it seems that no one has asked or addressed this question online. This is the one piece that is holding up my ADV 2A, so I'd really appreciate it if someone that knows how this works could fill me in. Thanks in advance!