What is the << (double left arrow) syntax in YAML called, and where's it specced?

Question

The <<: operator in YAML is usable to import the contents of one mapping into another, similarly to the ** double-splat operator in Python or

Answer 1

It is called the Merge Key Language-Independent Type for YAML version 1.1. and specced here

It is something that parsers can optionally support, it is essentially an interpretation of the key-value pair with the special key <<, where the value is either a mapping (usually indicated via an alias as in the spec, and although that doesn't seem to be required, it makes little sense not to use an alias) or a list of mappings (i.e. aliases of mappings), and gets interpreted in a special way.