"efficient scalable code" is just as vague as good code. How are you going to know your code is scalable if you don't understand your domain? Scalability is not something you sprinkle onto code.
Sometimes you need two orders of magnitude. Sometimes you need five. Sometimes you need less than one.
Efficiency is about using minimum cpu cycles or minimum memory or minimum network round trip or more generically using minimum/optimum resources to get something done.
Scalability is about minimizing bottlenecks and linear scaling so one can just copy and execute by adding more nodes/resources and expect correctness and increased throughput.
Both of these have nothing to do with domain expertise.
No profitable business wants to pay you for writing code that uses "a minimum" of a resource. It wants to pay you to find the right balance between resource usage, time-to-market, operating cost, code complexity and probably several other factors.
Yes, technically, but note that this entire thing can be anything from crucial to completely worthless depending on the domain.
You need insane scalability for a social network or a streaming service, you don’t need any real scalability for (completely made up) managing a fleet of airplanes or the internal logistics of a zoo.
But I'd also claim that these things fall on a spectrum. At the extreme end we have exchanges and HFT-like trading systems, where absolute accuracy and latency are not even constraints but industry fundamentals. At the other end we have "toy" applications that handle tens of requests per second, tops.
Scalability problems are definitely near the extreme end. Only instead of raw latency, you get to deal with complex failure modes, throughput, capacity problems, read amplification and thundering herds... all the while being constrained by available CPU cycles and bounded memory.