Japanese grammar/kanji rule info

Ok this has bothered me long enough that I cannot lurk no longer because I’m not even sure how to ask the question to look it up. I’ll have to give examples.

おおぞら　　大空　
　空
*in a dictionary it’s そ. Where does ぞcome from?

This is everywhere! And not usually in the dictionary for the word.

人々　the duplicate kanji.. it’s supposed to be a duplicate but it’s not exactly the same. Oh nooo. It’s hitoBito.

I know it makes it easier to speak and say… I’d just like to know what this is called so I can learn about it. I’ve read numerous beginner books and I’ve never ran into anything elaborating on this -that I can remember.