There are two sets of binaries available on AdoptOpenJDK site now:
OpenJDK binaries - this is visible in the main part of the web page. Those are build using scripts from AdoptOpenJDK/openjdk-build repository.
OpenJDK upstream binaries - there is a small section below, with "AdoptOpenJDK now also distributes OpenJDK upstream builds! (Built by Red Hat)", linking to this page. Those are build using AdoptOpenJDK/openjdk11-upstream-binaries repository.
The latter build is easy to understand, it just clones appropriate tag from OpenJDK repository and just runs OpenJDK build script. BTW. it was quite surprising to see that although OpenJDK do not provide binaries for example for OpenJDK 11, it still maintains it:)
Now the question is what is added in the AdoptOpenJDK OpenJDK binaries, on top of plain OpenJDK upstream binaries?
NOTE: basic information about how JDK/OpenJDK/OracleJDK/etc relates to each other may be read in Difference between OpenJDK and AdoptOpenJDK.