1. What do the double parenthesis '(( Values .... ))' mean?
Nothing, it was easier to continue typing and close one more on the phone. Single parenthesis are enough.
2. What is the 'x(field)' in the column alias?
X(field) is just to make the example more readable. And name the example table x with the column field to illustrate the main part of the example
encode(yourfield, 'hex')::macaddr