So is Aurora just a MySQL storage engine (e.g., InnoDB), or a modified version of MySQL under the hood?

It's api is based on mysql, doesn't share interals with it.

They had a deep dive into during the last re:invent.

https://www.youtube.com/watch?v=CwWFrZGMDds

Aurora is a modified MySQL 5.6.10. It shares several bugs with 5.6.10 that have already been fixed in later versions of MySQL. My understanding is that it mostly remains unchanged other than the massive enhancements Amazon has made to the I/O performance.

Sure it does. You can see this from diagnostic commands (Innodb status, mutexes etc)

Really? Werner Vogels said they built a totally different logging and storage layer.

Also a good bit of video of Vogels talking about Aurora.

https://www.youtube.com/watch?v=0rPpCnFE-hU&feature=youtu.be...

Logging and storage being new does not mean it does not share a lot of other internals.