@Entity
public class User {
    //...
    @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Note> notes = new ArrayList<>();
    //...

    public void addNotes(Note... notes) {
        for (Note note : notes) {
            note.setUser(this);
            this.notes.add(note);
        }
    }
}

@Entity
public class Note {
    //...
    @ManyToOne
    private User user;
    //...
}

User user = new User(...);
user.addNotes(new Note(...), new Note(...));
userRepo.save(user);