Only core Java. Knowledge of functional programming in Java (lambdas) expected. No Java GUI experience (such as swing or AWT classes) required. No knowledge of specialized areas in Java (such as familiarity with Java EE, Java ME versions, JDBC, Spring & related technologies, hybernate and so on...) required.
It will be added advantage if you are familiar with Vaadin https://vaadin.com/. Vaadin is used to build the UI in SO Platform but don't worry if you are not familiar with it yet.
Very basic concepts only.
IntelliJ IDEA (Community edition will do. However, please make sure that you have the latest version).
This should be the OS on your development machine (desktop/laptop). Ubuntu will do (Recent version is 20.04 or later).
Please start reading the wiki pages: SO Training Wiki Pages