Wisdom I got many years ago from an old hand was to run this thought experiment:
* An HTTP request
* A business object
* A database transaction
That's web architecture, more or less. Someone requests something and your programs mission is to find that something as fast as possible. In some cases Rails is a great fit, depending on the constraints. Under different constraints Go is the right solution.
When learning the new technologies the key is understanding what problem that technology is trying to solve most optimally.
Additionally, what really separates a "Senior Software Engineer" from the levels above is understanding that after a certain point, technology is irrelevant. Social factors drive decisions much more than technical considerations (Conway's Law).