Where does Gradle output the build jars?

Depends on which you talk about and what you maybe configured. If you mean what the

jar

task produces and you did not do change defaults, then in

build/libs

How do we set it up to output jars?

It's totally unclear to me what you are trying to ask

How do I configure it to output jar files?

Still not clear. Practically any plugin you apply will cause a jar to be built

K