I’ve bought some sourcebooks in real life, and i don’t know if i can add them to my online library, if i can, how? and if i can’t then sorry for wasting your time.

Physical books cannot be redeemed on D&D Beyond. There are four ways to use non-free content on D&D Beyond:

• Purchase the whole book
• Purchase just the options from the book you need
• Join a content sharing enabled campaign with someone who owns the book/content
• Use the homebrew tools to copy the content into D&D Beyond for personal use only, do not publish

There is a very useful thread here on digital vs physical copies of books and how it relates to D&D Beyond