A lot of online software industry content is focused on junior engineers: algorithms and data structures, usage of SQL, etc... But I don't think we often discuss what really sets mid/senior engineers apart.
My list would probably look something like:
- ability to use a proper debugging stack (not just tests/printf, use of microbenchmarks for optimisation, systems tracing for services)
- optimisation of how their app interacts with the environment (kernel, network stack, cache lines)
- debugging beyond application level issues (e.g. ability to use core dumps, knowledge of how the JVM works)