So understanding what nodejs is important to understand when working beyond the front-end.
The front-end is essentially the html+css+js that is ran by the user’s browser. Nodejs essentially is another execution environment that runs JS, by also providing different apis such as accessing the file system.
Firebase is a Google Saas platform that provides a number of libraries and apis to make mobile and web development easier. There are a number of services that are available to help build your application how you need it.
If you want to leverage the platform to send emails for things outside of authentication (Firebase has this built in) your best bet is to use Firebase functions, which builds ontop of Google Cloud’s cloud functions, except provides some nice features to integrate with Firebase authentication out of the box, so you can more easily integrate with the function in your Angular project.
You can add your secret’s/configurations in a cloud function to send emails using mailgun as-is. Then you can call this function from your Angular project when required (or via other triggers, like from changes in Firebase)
I personally recommend going back to basics before going further into more tech. Angular is not an easy framework to learn, due to its expansive api and opinionated approach to building apps. Stuff like dependency injection, and rxjs (Reactive programming lib) are used heavily within Angular, and are complex systems in their own right. Throw in Firebase, and its easy to get overloaded and lose sight of the core fundamentals of why and how things are actually working.
If you do want to push forward with Angular and Firebase, just be sure to be careful with using Firebase as its a pay to use service, and easy to totally crew yourself over, there are plenty of horror stories where you are getting charged thousands of dollars per hour.
On resources, I recommend being able to go thru the entire tour of heroes app on the Angular docs. Other good resources are https://fireship.io/, and https://www.learnrxjs.io/. The Angular docs (linked above with the heroes app) are always great to learn fundamentals and techniques specific to Angular.
Regardless of what you do, good luck!