Hi @Roxie423 - most of these points can be answered by the corporate/company IT services team. Larger companies will usually use or require Federated logins via Azure AD etc as part of any external integration. Some companies have security policies that require all SaaS offerings use SSO where possible as it helps keep company & customer data secure by disabling service logins (like Experience Cloud) when the directory user is disabled. If you use Adobe ID's then its possible that the user can access the services after they leave the organisation which can be a security issue.
So check with your IT or security teams as to what the policies around identity management are for external services and you should have most of your your questions answered.