Receiving Rabbinic Ordination

Regarding the response to my question in (https://sheilot.com/answers/view/10504/) A. Is there no source? B. Today, after the Rebbe's permission, "a time to act," even the Oral Torah can be learned from books. C. Is there really no source or drasha for this, and is it just the esteemed rabbi's opinion? Thank you, continue to increase the soldiers for Torah and testimony.

Peace and blessings 

A. I have attached sources in the 'Source' tab.

B. There are many details that cannot be learned from books, such as colors and appearances. Mainly, analogy cannot be learned from a book, but from a rabbi who will say when the matter is similar and when it is not.

C. In the Talmud, there are several places where it is said that it is forbidden to instruct without the use of sages, (such as Sotah 22b, Chullin 44a), but as you noted, 'use' in the language of the Talmud refers to the entire Oral Torah. Therefore, the drashot do not fit our case.

Darkei Moshe, short, Yoreh De'ah, Siman 242
Ribash in response, Siman 271, wrote that the ordination practiced in our time is not so that the ordained can judge, and if he errs, he will not pay, as this permission is only effective from the head of the exile or from one who received permission from him. And even if we say that the head of the exile gave permission to others, and others to others until our time, this does not matter since in our time the head of the exile is nullified, also his representatives are nullified, as stated in the tractate Gittin (29b) all of them come from the owner, etc. And it can be said that this ordination is when the student has reached the level of teaching, and by law, he is allowed to teach outside of three parsahs, and even obligated to teach, but they decreed that a student is forbidden to teach unless he received permission from his teacher or his teacher gives him permission to establish a yeshiva anywhere and teach and instruct everyone who comes to ask, and this is when he is called a rabbi, meaning now he is as if he is not a student, but worthy to teach others anywhere and be called a rabbi. And if not in this way, I see no reason for this ordination. And yet it seems that if not during his teacher's lifetime, he does not need permission from the teacher, as the decree of Rabbi was only during the teacher's lifetime, but not after his death, as they did not decree otherwise than what was in the first chapter of Sanhedrin (5a), and therefore if his prominent teacher died, he does not need any permission. And a fellow student does not need permission, as they decreed only for a prominent teacher. And therefore, I am extremely amazed at one rabbi named R.M. Levi, who decreed that anyone who does not receive permission from R. Yeshaya, his divorces and halitzot will be invalid, and if they are done properly and according to the sages, they are valid by law, why should they be invalid and why is permission from a rabbi needed for writing and giving a divorce, as well as for halitzah. And if because the sages said (Kiddushin 6a) anyone who does not know the laws of divorces and betrothals should not engage with them, ordinary scribes of judges learn (Gittin 2b). And what they said not to engage with them, Rashi explained, to be a judge in this matter, lest he permit a forbidden relation, but if the divorce is written properly, why should it be invalid, and ordinary scribes of judges learn and have corrections of the divorce in all its laws, as the authors, may their memory be blessed, ordained.
However, in the response of R. David Cohen (Siman 22) (Beit Yach, Room 10) it is written that in our time it is customary to ordain so that it is known that he is worthy to teach, and not every student will teach, and therefore it is customary to ordain so that now it is known to all that anyone who is not ordained has not reached the level of teaching, and he cannot be relied upon unless it is known to all that he is a great man and due to his humility does not seek greatness or for other reasons. And my heart worries that all divorces given in our time without an expert for many, that is, whose name is known by the sages, that he is a great man, should be concerned about them. It is also customary to ordain because a student is forbidden to teach in the presence of his teacher unless he received permission, and how will it be known that he has permission, because he is called a rabbi. And all this is not like the author of Nachalat Avot, who wrote in the chapter "Shanu Chachamim" that he does not know the reason for ordination in our time. And Mahar"i Weil wrote in response, Siman 85 (and 122) [and 128], that if he did not receive permission and gave divorces, even post facto it is not a divorce, and we do not say in this case that ordinary scribes learn, etc. And according to all opinions, if it is known and recognized that he is learned, but did not receive permission, his divorces are valid:
Rambam wrote in the laws of Sanhedrin, chapter 4 (halacha 8), that the court has the right to ordain for individual matters, and this is if he is worthy for all matters, as a distinguished sage who is worthy to teach the entire Torah, the court has the right to ordain him for some matters, such as judging (monetary matters), but not to teach prohibitions and permissions, etc. And it seems that this is only in their times when the law of ordination was practiced, but ordination in our time is nothing but mere permission, and for one who is worthy, he is worthy.