Comment by hybrids

As Half-Life 2 itself did not have a developer commentary, I personally recommend checking the book "Half-Life 2: Raising the Bar," and, if you really want to do a deep and lengthy dive into the particulars of HL2's level design, check out Robert Yang's commentary-driven playthrough of HL2: https://www.youtube.com/watch?v=VheYcG2T5qw