I want to continue my story about howa business you could invest in a business if you had all the money in the world. I want to continue my story about a business you could invest in a if you had all the money in the world.

This is a minor difference in meaning. The original sentence sounds like you are investing in business in general. The correction makes it clear you are talking about investing in a specific business.

I would like to change the textbooks. I would like to change the textbooks.

Better: I would like to change how we do textbooks

But how to organise all this in a legal, easy, and efficient way so that the teacher can choose different parts of different books and build histheir own digital book? But how to organise all this in a legal, easy, and efficient way so that the teacher can choose different parts of different books and build their own digital book?

This use of 'he' here is a little old fashioned, and modern style guides are more likely to suggest using 'their' or 'his/her' for a singular pronoun that could potentially refer to either a man or woman. More importantly native English speakers wouldn't naturally use 'he' like that, it's something artificially enforced by old grammar rules. If this is what your teacher taught you to do or is telling you to do and you need to follow it to be corrected as right that's fine, but just letting you know so you are aware.