Retirement tax questions

I defer to @dmertz  in all things. I’m sorry that I did not clarify that over age 59 1/2, the rules are very different.  The treatment of the rollover before age 59 1/2 as separating contributions from earnings makes logical sense, but I could not find it documented anywhere in publication 590Aor 590B, and I still don’t understand where that rule comes from.