Ihr is the plural of du
But in english they are both "you"
There are afaik 4 translations for "you":
Du/du - addresses one person informally
- David, you are my friend. - David, Du bist mein Freund.
ihr - addresses more than one person informally
- David and Robert, you are my friends. - David und Robert, ihr seid meine Freunde.
Sie - addresses one person formally
- Mr Peters, are you learning German? - Herr Peters, lernen Sie deutsch?
sie - addresses more than one person formally
- Mr and Mrs Peters, are you learning German? - Herr und Frau Peters, lernen sie deutsch?
The last one is wrong. "Sie" as formal address is always capitalized, regardless of singular or plural. 'sie' with lowercase 's' can mean she or they, but not 'you'.
Ihr is closer to our "you all" or "y'all." You can consider du and Ihr analogs of the Old English words, thou and ye, which we used for the informal you, and for both the singular formal and plural you, respectively. We have since replaced all forms of the 2nd person nominative (when it is the subject of the sentence) personal pronouns with just "you."
Ellos isn't the conjucation for "All of you". It's the 3rd-Person plural. Ustedes (Also Vosotros in Spain) is the 2nd-Person plural
To give another example of a plural "you" that still survives in English, you'll often hear "youse" in Scotland to refer to many. It's a pretty useful bit of slang and some of my workmates have even adopted it for its specificity!