Jenkins is Java based and requires a JVM (or JDK) on various environments and customers (jenkins users) are quickly lost about them.
There are several JVMs to consider:
1/ The one used to host the Master (or OC). This one depends of the system and could be installed using a native package or something like that. Then Jenkins Master must be configured to use it.
2/ The one used on each agent to launch the agent. Like the master it could be installed at the system level and the agent could be configured to use it. But you have also some kind of agents which are installing the JVM for you (SSH agents ...). It's minimum version is the same than the master but we advice to use the same level of JVM on the master and all agents.
3/ The JDK (more than the JVM often) used to run the builds. This one could be installed/configured by jenkins and at the job level. It depends of the kind of the job but in theory any version of JVM can be used here (excepted the Maven Job Type which requires the min version supported by the master).