Dangerous assumptions and falsehood has to be dealt with by the answer of this post. This question is very important and thank you for making it.
You will start learning regularly about a subject. Then, proficiency will come. The Demon will give you insights, warnings, guide you to proper knowledge. I underline here proper, because there is a lot of improper knowledge, which Demons can help in any subject for one to not fall onto.
They can give immensely propelling insight, but there is more into this on the practical level. Anything they give, will be true. If it's not true, then it's wishful thinking.
This growth will be at a growing pace and so on. The Demon will help you attain greater understanding, but this is done through your own mind. Eventually, what you learn in 3-5 years, others may take 10 or 20 years to learn. Or what others might "never" learn.
As you rise and rise, he will give you more clues and on the final stages, direct knowledge.
A lot of people have been unknowing guided in the Demons in their fields. This can lead to extraordinary excellence in a subject.
Paganini is an example where the Violin is concerned. One interested here should look into Paganini. Paganini was a favorite child of the Demons, a loyal disciple. Not only a born prodigy, but one that pleased the Gods in learning. He was hardworking in his craft, and became legendary. This might take more than one lifetimes.
Next life, one will also have a headstart because of the effort invested.
Is a Demon standing at you and teaches you like at school or He blesses you with a knowledge of medicine? - This can take both forms, with one building on top of the other. But to reach the late stages of direct teaching, one has to be a student first, like in a university type of rising to a level.
The nuclear physics ultimate university professor cannot teach untalented or non prodigal students that do not know the basics of physics for example. One has to be to a level, then move onwards.
Demons teach the perfection of subjects, compared to sloppy "results" and sloppy "learning" that is not going to lead to proficiency. This however entails an effort and for one to stay on something if they truly want to learn it.
They guide to the proper knowledge and the knowledge of the Gods on a higher level, and the above describes the path this takes and the form under which the teaching occurs.
Lastly, and most importantly: Go with your experience here, ie, approach them to that end.
-High Priest Hooded Cobra 666