I would consider movement within Amazon to be more difficult than movement within Alphabet as you have to interview again for your new team at Amazon, or at least this was the case when I was there.
(I work at Google and previously worked at AWS)
Alphabet wasn't broken up. It is 1 company. There is a difference between reorganization within 1 company and breaking up a company into separate companies. Not sure how else I can explain it.
Standard Oil was broken up into separate companies - which eventually become exxon,amaco,mobile,chevron,etc. Alphabet just reorganized their corporate structure. Meaning after the reorganization, google, youtube, deepmind, etc remained part of alphabet. Exxon, Amaco, Mobile, chevron, etc were not part of standard oil after the break up.