Question: I've never seen the show so an explanation would really help. Why would Johnny open up his own Cobra Kai dojo? Since there was probably a lot of negative publicity due to how Kreese taught his students and how he attacked Johnny, a lot of people probably wouldn't go to a dojo synonymous with brutality but maybe to a dojo that taught karate the way it should be learned.
Answer: Just like in the original film, some guys want that brutality. Johnny in part opens the dojo because of his past rivalry with Daniel and to train a kid that's being bullied. But also, other kids that are being bullied want to join in order to learn to fight back. Johnny is also teaching the Cobra Kai creed (Strike First. Strike hard. No Mercy.") so that's why he opens his dojo with that name and the old Cobra Kai dojo's past reputation is exactly what he wants to elicit.
Question: Johnny has a son named Robbie. Johnny seems to want nothing to do with him so why does he keep a picture of him on his refrigerator?
Question: What kind of disease did Tommy (one of Johnny's friends) succumb to?
Answer: It isn't said specifically, but Tommy does mention that "it's in here now" while pointing to his head. This suggests some sort of cancer has spread to his brain.
Answer: It's implied to be a combination of brain damage (Johnny suggests this to Miguel earlier) and cancer that spread to the brain. In real life the actor suffered kidney and liver failure and died not long after.
Question: How come Sam and Aisha stopped being friends just cause Sam was hanging out with Yasmine?
Answer: Yasime was being mean to her.
But Sam wasn't. It was even obvious that Sam didn't like it.
Sam and Aisha were long-time friends and Sam wasn't standing up for Aisha at all. She continued being friends with Yasmine even though she was being cruel to Aisha. Being uncomfortable at Yasmine's insults is not going far enough to stand up for Aisha.
But Sam also didn't come to her defense either. So that's a good reason for Aisha to not like her.
Answer: Sam picked her prettier and more popular friends over Aisha. They still got along until Yasmine publicly humiliated Aisha.
Question: Silver tells Chozen he's never had any children himself. If that's the case, then how are people calling Snake from part 3 Silver's son?
Answer: I've seen the 1989 movie many times and never once did Silver ever present Snake as his "son." Snake and Dennis were two young men who simply worked for and/or lived with Silver and only called "the boys" when Mike first arrived.
I never said it was said in the movie. I said people are saying it. Social media. Etc.
Simple, people don't know anything and assume baseless things for absolutely no reason.
Answer: When the movie came out in '89, it was revealed that the director's son had a part written just for him as the villain's son, and he would be called Snake. This relationship, however, was never addressed in the film itself and Snake just became a member of the house staff for Silver.
Question: Is the actress who plays Robby's mother in season 2 the same actress from season 1?
Answer: Yes, it is the same actress, Diora Baird. The character's name is Shannon.
Question: I thought Johnny and Daniel patched things up at the end of the first movie? So why do they still hate each other?
Answer: The first episode really only shows that Johnny still hates Daniel. Daniel is shown being friendly with Johnny when Johnny visited the car shop. Johnny was down and out and he was most likely jealous of Daniel's success. Only when Johnny was still giving Daniel a hard time did Daniel start feeling the hate again.
Answer: Actually, at the end of the first film (and by extension the beginning of the second), the only interaction they have is when Johnny presents Daniel with the trophy and compliments him on the match. Daniel never intervenes in the parking lot confrontation, and there is no indication that Johnny and Daniel have become friendly to any degree. This does not necessarily mean they've patched things up. Also, the previous answer that points out Daniel's initial friendliness and Johnny's hostility is correct.
Answer: Both.