ok this is straight from my spiderman ultimate guide..SPAWN OF THE SYMBIOTE:CLETUS KASADY WAS SENTENCED TO LIFE PIMPRISONMENT AT RAVENCROFT PRISON, A MAXIMUM SECURITY INSTITUTION FOR SUPERHUMAN CRINIMALS. THERE HE FOUND HIMSELF SHARING A CELL WITH EDDIE BROCK. BROCK, WHO HAD ONCE BEEN KNOWN AS VENOM HAD RECENTLY SEPARATED FROM HIS ALIEN SYMBIOTE. ONE NIGHT, THE SYMBIOTE RETURNED TO FREE EDDIE, AS CLETUS WATCHED IN AMAZEMENT. i'm not sure if thats the answer.. but wait let me seeee..aha! ok i guess so after eddie breaked out of jail cletus wass left in shock.. blah blah blah umm however a small part of the alien symbiote that had been left behind merged with kasady. the combination of the symbiote and cletus' dangerous mind formed the psychopathic killer called CARNAGE!!

Basically a criminal, Cletus, shared a cell with Eddie. Symbiote returns to eddie, and has a baby symbiote in the cell. Venom forgets about it, the baby attachs to Cletus, and so Cletus breaks out and starts killing people. Venom doesn't like having a child that doesn't obey him blah blah blah so that's why both spidey and venom fight carnage. Carnage of course gets squashed by spidey at some point and loses the symbiote. However, he makes an identical suit but still has no powers.

As far as I know.

Yes because venom created carnage

Yes because theyre both made of simbiots