I do think they learn some basics - about the nature of the warp, demons, etc. but only what they are allowed to know, of course - what would help them to be careful, to remain stable.

Heinrix in Voidshadows DLC responds on the idea that he could do the ritual:

"He is right, {name}. I know some of the fundamentals, but this isn't my primary field."

It could be something Heinrix specific, but it is also possible that psykers learn at least some basics about other fields - maybe just that they exist, how to recognise them perhaps.

On Janus, Heinrix senses psychic powers and know it is a bewitching spell - but - it is possible of course, that he just sensed something and given the reaction of the rebels, came to the logical conclusion it has to be mind affecting.

"These people are not thinking straight, {name}." Heinrix stares at the tall, robed figure without blinking. "I can sense an influence. A bewitching spell. Sorcery known only to the strongest of the Aeldari — their Farseers."

Now, even given the sanctioning, training and, for sure, brainwashing that is going on there, I don't think every psyker is that devout and has such a strong belief that is required to make the sanctic discipline work.

You have to believe that it works, that the God-Emperor really protects you, to make it efficient and to make it work. So whenever Heinrix says (and he says it a lot) - God-Emperor protects - that isn't just a phrase; he actually believes it.

And in general, we do see him very much being devout, believing that their escape from Commorragh was a miracle sent by the God-Emperor, or in Voidshadows he isn't doubting that it was the Emperor sending them a warning.

So I think he already had strong faith, because I don't think that if he hadn't, Xavier simply asking him to believe would have worked that well. And since he said "master", it sounds to me more like he already used the powers of that field, but there was potential for more (basically, the higher-level powers).

And I assume he learned that during the training/sanctioning when they identified him as a potential candidate for that discipline. As a battle psyker in the Imperial Guard, he might already have had opportunities to use them.

Xavier then encouraged him to master them because they are so incredibly useful. They work against Chaos, as well as against enemy psykers and xenos. And in his service to the Inquisition, there was for sure no shortage of situations in which to use them.

My personal head canon is that the psykers in the game would not start with these few powers, that this is simply a gameplay decision, but instead would already wield higher level powers as well. I do think they can get even stronger over time; I think it was also said that through the warp rift more and more psychic people are born and that psychic powers are now increased, but maybe that is just for those worlds near the rift.

I hope that was what you were looking for - sorry, there isn't that much about it in the game, as you also already noticed.