These jobs are for building OpenJDK source code into functional JDKs for a variety of platforms. See https://adoptium.net for the homepage of this initiative and https://github.com/Adoptium for the source code.
Build jobs have names that look like "openjdk_build__". Each job will take a copy of the latest source code mirrored in Github, pull in the build dependencies, and execute a build. The Jenkins job to do this is minimal, as we prefer to keep all the logic for building a JDK in scripts. These are available in the Adoptium/temurin-build repository.