How does Garbage Collection work internally?

GC reclaims objects that are no longer reachable from GC roots (thread stacks, static fields, JNI references).

Modern collectors are generational, built on the weak generational hypothesis: most objects die young. New objects are allocated in the young generation's Eden; a minor GC copies survivors between survivor spaces and ages them, and objects that survive enough cycles are promoted (tenured) to the old generation. Collection uses mark-sweep(-compact) or copying algorithms, often with a Stop-The-World pause to walk the object graph, though concurrent collectors do most marking while the app runs.