Sub Broker / Dealer Terminal / Trading Multiple Accounts

Most old school brokers provide a dealer’s terminal via sub brokers to enable trading in multiple accounts with one login. I understand none of the online discount stock brokers including Zerodha work on that model. So what is the equivalent of it in Zerodha.

How can one (RIA) trade on multiple client’s accounts simultaneously.

I think RIA can’t trade on behalf of clients, he can advice them but the order should be punched by client or there should be confirmation from client via email or phone call to place order.