Building Eclipse itself is horror, because it is not at all clear what needs to happen to actually develop and build it. This should, once, become a reasonable description of how to - for instance - add a fix to the compiler. But for now it's just a list of links and snippets.
Building a subdirectory from the Aggregator clone:
cd eclipse.jdt.core/org.eclipse.jdt.core mvn -Pbuild-individual-bundles clean verify
If JUnit tests fail out of the box you can skip them using:
mvn -Pbuild-individual-bundles clean verify -Dmaven.test.skip=true -DskipTests=true
To build the batch compiler issue that inside eclipse.jdt.core, then find the compiler under org.eclipse.jdt.core/target/org.eclipse.jdt.core-3.14.100-SNAPSHOT-batch-compiler.jar