How does @Transactional isolation work without locking?

Question

Figured out from here: https://stackoverflow.com/a/58831350/10894456 @Transactional doesn't lock rows triggered on entities which are used inside @Transactional methods.

And one need to use either optimistic ( @Version annotation in Entities) or pessimistic ( @Lock in repositary methods) explicit directives out of the @Transactional method.

The question is: how does isolation work on @Transactional methods in the face of any locks absence? e.g.

@Service
public class AService {
    @Autowired
    private Repository1 repository1;

    @Autowired
    private Repository2 repository2;

    @Autowired
    private Repository3 repository3;

    @Transactional(isolation = Isolation.SERIALIZABLE)
    public Result getResult(Long id) {
        Entity1 e1 = repository1.findById(id);
        Entity2 e2 = repository2.findById(id);
        Entity3 e3 = repository3.findById(id);
        e1.setField(doSomeLogic(...)));
        e2.setField(doSomeLogic(...)));
        e3.setField(doSomeLogic(...)));
        repository1.save(e1);
        repository2.save(e2);
        repository3.save(e3);
        return Result.combine(e1,e2,e3);
    }
}

What is guaranteed by using

@Transactional(isolation = Isolation.SERIALIZABLE)

? And how do this guarantees work without any locking?