As somebody who's not deep in the Java ecosystem, the precise relationship is not clear to me, but it does seem that Oracle is a major contributor to OpenJDK at least, most likely also involved with the project steering.
I don’t think this is accurate. Alpine and Debian OpenJDK have been getting point releases (u###) through the distribution package manager. I’ve got apps running on openjdk 8 and 11 (the LTS versions) and they are still getting u### releases with security fixes and bug fixes despite Java trunk already having moved past 11.
Edit: did some further research on this and it looks like various distros are providing the LTS support for java 11. For instance Amazon Linux will be supporting Java 11 through 2023. https://dzone.com/articles/oracle-continues-long-term-suppor...