Temurin and others are "distributions" of OpenJDK, basically their compilation results of it, not their own codebase. They're not "forks" in terms of source code, but they have patches, build systems, QA, and everything else around it that they apply, then offer you their version of it.

  OpenJDK: where Java is developed
  Temurin / Zulu: where OpenJDK is built, tested, packaged, and supported