The JDKs supported
This is a commercial version of the brand with paid support. It can be downloaded and used free of charge for development use only. It cannot be used in production without paying Oracle (so it's a trap for the unsuspecting). Oracle intends to provide full, paid support until 2026 or later. Note that, unlike the past, Oracle JDK is not "better" than the OpenJDK build (as long as both are at the same level as the security patch).
These are free and unbranded versions of the OpenJDK under the GPL license with Classpath Exception (secure for business use). These builds are only available in the first six months of a release. For Java 11, the expectation is that there will be Java 11.0.0 and then two security patches, 11.0.1 and 11.0.2. To continue using the OpenJDK build by Oracle with security patches, you would have to switch to Java 12 within a month after the launch. Note that the provision of security patches is not the same as support. Support involves paying someone to do the sorting and acting on the error reports.
These are free and unbranded OpenJDK builds under the GPL license with the Classpath Exception. Unlike Oracle's OpenJDK builds, these builds will continue for a much longer period for major versions such as Java 11. The versions of Java 11 will continue for four years, one year after the next major release. AdoptOpenJDK is a community group. They will provide builds as long as other groups create and publish security fixes to a source repository in OpenJDK. Both IBM and Red Hat have indicated that they intend to provide these security patches.
In addition to standard OpenJDK builds, AdoptOpenJDK will also provide builds with OpenJ9 instead of HotSpot. OpenJ9 was originally the IBM JVM, but OpenJ9 is now open source in Eclipse. This one, in fact, is something to be studied.
Red Hat provides OpenJDK builds through Red Hat Enterprise Linux (RHEL), which is a commercial product with paid support. They are very good at providing security fixes back to OpenJDK. In the past, Red Hat has run the Java 6 and 7 security update project. The Red Hat build is better integrated with the operating system, so it is not a pure OpenJDK build (no end-user JDK)
Zulu is a branded version of OpenJDK with paid commercial support. In addition, Azul provides some Zulu constructions for free as part of the "Zulu Community;" however, there are no specific commitments regarding the availability of these free constructions. Azul has an extensive plan to commercially support Zulu, including plans to support Java 9, 13, and 15, unlike any other vendor.
Among all, it is the newest option. Corretto is a no-cost build of OpenJDK with long-term support passing through the TCK. It is under the standard GPL + CE license of all versions of OpenJDK. Amazon will add its own patches and run Corretto on AWS, so it will be heavily used (it has already been placed on some products). Support for Java 8 will be at least until June 2023.