How to write a number in C in "decimal code"?

Question

C++ / C#

Please help me understand ' ', " " and decimal code of characters.
I read the book, but then everything was thrown into a heap and I did not quite understand.
They write that '0' is the value 48, which has nothing to do with 0.
More different examples, but
When in C should you use ' ', and when " " and what's the difference?
And how, using decimal encoding, where '0' is 48, with these numbers that refer to the table of these "48", write a word with these numbers or write numbers with them and add them?

Answer 1

everything is here :)