How much does Meta expand your audience?

One common point of confusion for Meta advertisers is Meta’s expansion of ad targeting. In some cases, advertisers don’t realize it’s possible at all (but is). In others, it’s completely unclear how much an audience was expanded.
To make things worse, Meta has added new targeting features related to audience expansion that sound similar but are different in subtle ways.
The concept of targeting expansion began with Advantage Detailed Targeting (originally Detailed Targeting Expansion). That same approach was applied to Lookalike Audiences ( Advantage Lookalike) and even custom audiences ( Advantage Custom Audience).