Hey @Muni - you are asking all the great and valid questions when it comes to being new to EDA! First off, welcome to the Solace Community.
I would like to step back and take a higher perspective on EDA before addressing your questions. When it comes to building and using APIs in general (whether asynchronous Event APIs or synchronous REST APIs) you should take into account two main aspets: Design-Time and Run-Time. In an extremely simplified manner:
Design Time: This involves all the activities and tooling required to design, govern, document, collaborate, share, expose, and manage your APIs. For example, API Portals in the REST API world fit in this category
Run Time: This involves all the activities and tooling required to actually run and interact with your APIs. For example, in the REST API world this would be an API Gateway.
To read more about this check out this blog https://solace.com/blog/api-management-event-management/
Now from an EDA perspective, you can think of the Event Portal (EP) as your design time component where you can design your event driven architecture. And the Event Broker as your run-time component of the EDA system where event streaming and data movement (i.e. pub/sub) occurs. Note: Pub/Sub is one of many message exchange patterns in EDA!
It is important to note that while the Event Portal is primarily used (and promoted) as a design time tool for EDA, it is envisioned to be more than that. We are on the path to closing the loop between design and run time through the introduction of Discovery and Audit in EP that will marry the two worlds together. Infrastructure provisioning of the broker from the the portal is also on the roadmap.
Another way to marry the two worlds of design and run time in EDA is what @Aaron mentioned → through the use of AsyncAPI you can install application specification from EP and use code generators to translate your designed spec file to a runnable runtime application. A simple google and YouTube search will expose a good amount of resources on AsyncAPI, code generation, and other details that will help. To relate this to the REST world, AsyncAPI to EDA is what OpenAPI is to REST. There are alot of talks and resources on Youtube that will help with explaining those concepts as well.
Hopefully this helps with clearing out some of the confusion you had and get you on the right path for your EDA journey!
P.S. you might also find this blog post useful if you are a developer new to EDA https://solace.com/blog/developer-guide-event-driven-development/