A possible cause for why the portfolio manager cannot see ARC when trying to set up relationships between two accounts is that he has not been given the permissions that enable users to view and manage ARC. To access ARC, users need to have certain permissions and settings enabled in their user profile or permission set, such as:
View Setup and Configuration
Customize Application
Manage Custom Permissions
Financial Services Cloud Standard User
Financial Services Cloud Admin If any of these permissions or settings are missing, users may not be able to see or use ARC properly. Verified Reference: : Salesforce Help Article 5

Explanation
A suitable use case for Flow Builder is to collect input from internal users with a form placed on a Lightning page or launched by a button. Flow Builder is a tool that allows users to automate business processes by building flows that guide users through screens, execute logic, manipulate data, and interact with Salesforce and external systems. By using Flow Builder, the system administrator can create a flow that collects input from internal users with a form and performs actions based on the input.

Explanation
A gap analysis is a key step in identifying different business processes and outlining encompassing system flows based on the existing environment when setting up Salesforce Financial Services Cloud (FSC). A gap analysis is a method of comparing the current state of a system or process with the desired future state and identifying the gaps or differences between them. By conducting a gap analysis, a consultant can understand the customer's pain points, requirements, and expectations and design a solution that leverages the best practices and features of Salesforce FSC.
References:
Gap Analysis
Salesforce Financial Services Cloud Implementation Guide