Bible Criticism

Hello Rabbi, I have a brother who has gone astray and is trying to justify his ways because of biblical criticism. In fact, my brother is really an idiot, but he reads all kinds of nonsense and floods the house with it. I would appreciate a general (and if possible, specific) response to why biblical criticism is incorrect? What are the flaws and problems it suffers from? I would be very happy to see examples.
In fact, I'm kind of alone in thinking that this is nonsense because how do you observe "Yom Terua" and how do you observe "Remember the Sabbath day to keep it holy"? So I understand that this is not a serious matter, but I don't have the tools to respond.
In addition, the Rabbi, in the same matter, claims that the Torah was invented, and I try to explain to him that there is no way that a message that someone did, I did, and so on, can be implanted in history, but there is no way to convince people that they are today, they did "with you the Lord spoke," so he claims that perhaps they were taught that "with you the Lord spoke," etc. He is talking about the grandparents of the first recipients of the invention of the Torah, and in addition suggests that perhaps there was such an ancient legend, and one day the people of Israel decided to live according to that legend,,, I would appreciate the Rabbi's response.. because he is stirring up the whole family in the WhatsApp group!! and so on