Within the customer module you can manually indicate the agent of that customer. To do this go to the Commercial tab and in the "Agent" field indicate the one you want.
You can also perform a mass agent assignment to customers, for this you must perform an export of your customers and then perform an update from file.
Once the customers are assigned, the role configuration applied for their visibility is taken into account, and you can choose whether employees can see all existing customers or only those assigned to yourself.
To learn how to properly configure the roles you can review the help article on configuring user roles.