This is an excellent article outlining the impressive difference between the dreaded old audience expansion, and the new flashy optimized targeting.
In a quick summary, Audience Expansion was always a little opportunistic of google, to essentially say one audience selected from advertisers may be close in relationship to an audience you did not choose, so ads will be shown to them. The example in the article is, if you selected in-market SUV buyers, Audience Expansion would show ads to in-market car buyers because hey, they are kinda close. ¯\_(ツ)_/¯
But Optimized Targeting is totally different. It only looks at your conversion data and forms its own correlations to individual people after the machine learning finds a commonality between them. And ill repeat, ONLY CONVERTERS. so it sounds pretty great…