Yo quiero = quiero; Yo tengo = tengo
Saying "Yo" is optional. It is not constantly applied in typical conversation, as it is redundant.
It DOES come in very useful for added emphasis, however.
For example, in English we might emphasize like this:
You want ME to watch the baby?
Donald said HE paid for dinner?
WE raised our daughters to give the same respect they expect from others.
I worked for this family.
Saying all of this sentences without emphasis on the indicated word has a substantial impact on how the person hearing/reading will understand what is being communicated.
Using "Yo" can be utilized as such a tool for emphasis.
I love you. I love you.
I bought bread. I bought bread.
You and I were at the store. You and I were at the store.
Mi is singular: mi amigo/a. Plural: Mis amigos/as. (Also, no accent on the "i.") If DL allowed it, they were being nice (as in when I get something minor wrong, they give it to me and tell me I have a "typo." Aw, shucks, thanks DL. ) Interesting article here re the use of the gender-neutral "amigues" (!): https://perureports.com/opinion-amigos-or-amigas-the-new-trend-for-amigues-is-taking-over-the-continent/7956/