Erin Stellato had a lot of questions about Query Store, and the answers turned into a blog post:
5. If you have 3+ plans how does SQL Server decide which plan to use?
A: I assume this is specific to the Automatic Plan Correction feature, and if so, it will force the last good plan (most recent plan that performed better than the current plan). More details in my Automatic Plan Correction in SQL Server post.
6. What equivalent options we have for lower versions?
A: There is an open-source tool called Open Query Store for versions prior to SQL Server 2016.
Click through for all 19 of the questions.