How can I understand the JPA user manual more easily?

I'm new to JPA and generally understand what ORM technologies do. I'm having a serious problem, I want to understand the use of JPA from beginning to end in a comfortable way, instead of memorizing it. So, can you explain what was done here in order? Can anyone tell them what to do? For example, why are we creating the persistence xml file, can you tell me what should I do first and what should I do at the end? Also, could you explain what each line of code means below?

EntityManagerFactory emf = Persistence.createEntityManagerFactory("persistenceName");

EntityManager em = emf.createEntityManager();

EntityTransaction et = em.getTransaction();

et.begin();

em.methodOfEntityManager();
et.commit(); 

I'm having a hard time understanding the user manual for it.