1. Somebody/Someone/ Anybody etc
There is no difference between somebody and someone.
There is no difference between anybody and anyone
There is no difference between everybody and everyone
Nobody and No one.
We use singular and plural with these words

2. Which one to use in positive and negative.+ - ?
Today, everyone /everybody wants coffee
No one/nobody wants tea.
Does anyone/anybody want a biscuit?
There is someone/somebody in the room
There isn’t anyone in the room
Is there anyone /anybody in the house?

3. Which one to use in positive and negative.+ - ?
Someone/Somebody has opened the window ( I don’t know who)
There isn’t anyone/anybody with me
Is there anyone/anybody in the house?
Everyone/Everybody has left the house (all people)
There is nobody/no one with me
No one/Nobody came here with me
Not anyone/not anybody = No one, nobody
* (not anyone/not anybody is not the same as nobody/No one)