It's less complicated than you might think. A Java Development Kit (JDK) is a filesystem directory, and includes everything necessary to run a Java program. Most of the mysterious installers and version managers are managing a collection of these JDK directories in some fixed location on disk. You can download a JDK directory (tarball), and use the `java` binary within it directly.
There is also a convention of using the `JAVA_HOME` environment variable to allow tools to locate the correct JDK directory. For example, in a unix shell, add `$JAVA_HOME/bin` to your `PATH`.
There is also a convention of using the `JAVA_HOME` environment variable to allow tools to locate the correct JDK directory. For example, in a unix shell, add `$JAVA_HOME/bin` to your `PATH`.