On UA/CO, you can pay a bit extra and get Economy Plus, which has extra leg room. If you tend to be uncomfortable in Y (as I am, being tall), it makes long-haul flights slightly more bearable.

Supposedly passengers won't pay extra for comfort; I'm an exception.

When you book on the website, it will ask you if you want E+ and will tell you the price.

Otherwise, domestic Y is domestic Y.