What courses / books are best to take / read to learn how to create AR, VR, MR applications?

Question

augmented reality

I calmly programmed bots in Python, I wanted to switch to Node JS, but suddenly they invited me to make AR / VR applications on excellent conditions. But to work, you need to know. Unity and C# or another stack for developing mobile applications with AR or full-fledged applications for virtual interfaces. I would like to get links to the best courses / books on this topic. Preferably in Russian and for free.