JustJ will release p2 update sites for JREs derived from https://jdk.java.net. Specifically Java 11.0.2 and Java 14.0.2 for this release.
The update sites are documented here:
https://download.eclipse.org/justj/www/?page=download
The project provides extensive documentation about how the JREs are produced, their structure, the tools used to produce them, and how to use the JREs in Eclipse IDE/RCP products: