What is the difference between Aurora Mysql 5.7.x and Aurora 2.x.x
They're differences in the minor/patch versions of Aurora.
If you spin up an instance of Aurora (MySQL)-5.7.12, then run:
SELECT @@aurora_version; -- Returns 2.02.5

It's essentially the first version of MySQL 5.7 compatible Aurora.
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraMySQL.Updates.html