Did you look up the Gemara?
The Gemara says that Hillel said
“What you hate, don’t do to your friend. This is the whole Torah. The rest is explanation.Go and learn.”
So as to your second question, according to Hillel it is the whole Torah in a nutshell.
As to your first question. You missed out on half of Rashi. After Rashi explains that ‘friend’ is also HKB”H he says “don’t transgress His words [just like] you would hate if your friend transgressed your words”.
The last Rashi on this story explains that the rest of the Torah is an explanation of what HKBH’s words are that we shouldn’t transgress. For that Hillel said “Go and learn” what those things are (ie. the rest of the Torah).