Limiting "senior" to just technical aspect of tools or code is way too narrow to capture it's meaning. My all times favorite on the subject is this kitchensoap.com/2012/10/25/on-bein...
Also (my personal pet peeve) "senior" also thinks about why and for what purpose system is built. E.g. people working at Uber who not concerned about it's despicable business practices also show signs of immaturity.
That said this is not only about code.
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