How are textbooks meant to be referenced?

I'm studying maths, and I've found it beneficial to write my notes as if I were writing a textbook for someone else's self study - forces me to understand the material to some extent.

It's of course incredibly unlikely this could actually be made into a textbook, but hypothetically, if I decided in some distant future to publish a textbook on maths, what are the rules for referencing?

Is it acceptable to source material almost solely from other textbooks, and simply place in the references at the back the names of those text books, or...?

Exercises. If I borrow exercises from other textbooks which look challenging (not every exercise) then how required am I to reference the book I found the exercise? Even if I do reference, is there any issue that could stem from using those exercises? I've seen Wilson's theorem used as an exercise for almost every abstract algebra book I've read.

I know there are a few questions here, answers to any one of them would be appreciated.

The tags are an educated guess. This seems like a question for askacademia. Any suggestions for better tags or a better SE to put this question would also be welcome.