At what point is a transaction commited?

I've read about entities lifecycle, and the locking strategies, and I watched some videos about this but I'm still not sure I understand.I understand there is also a locking mechanism in the underlying RDBMS (I'm using mysql).

I would like to know at what point a transaction is committed / entity is detached and how does it affect other transactions from a locking point of view. At what point does an user have to wait till a transaction finishes ? I've made two different scenarios below. For the sake of understanding I'm asserting the table in the scenarios contains a lot of rows and the for loops takes 10 minute to complete.

Scenario 1:

@Stateless
public class AService implements AServiceInterface {
    @PersistenceContext(unitName = "my-pu")
    private EntityManager em;
    @Override
    public List<Aclass> getAll() {
        Query query = em.createQuery(SELECT_ALL_ROWS);
        return query.getResultList();
    }
    public void update(Aclass a) {
        em.merge(a);     
    }
}

and a calling class:

public aRadomClass{
    @EJB
    AServiceInterface service;

    public void method(){
       List<Aclass> listAclass = service.getAll();
       for(Aclass a : listAclass){
           a.setProperty(methodThatTakesTime());
           service.update(a); 
       }
    }
}

Without specifying a locking strategy : If another user wants to makes an update to one row in the table and the for loop already began but is not finished. Does he have to wait till the for loop is completed ?

Scenario 2:

@Stateless
public class AService implements AServiceInterface {
    @PersistenceContext(unitName = "my-pu")
    private EntityManager em;

    @Override
    public List<Aclass> getAllAndUpdate() {
        Query query = em.createQuery(SELECT_ALL_ROWS);
        List<Aclass> listAclass = query.getResultList();
        for(Aclass a : listAclass ){
           a.setProperty(methodThatTakesTime());
           em.merge(a);
        }    
    }
}

Same question.