Question: How did the streetcar, phone booths, and vending machines end up in the digiworld?
Answer: We are never told as to how they got there.
Answer: People who built the digital world most likely had them sent there. Gennai tells the kids in the last episode that the streetcar was from San Francisco, and he asked them to take it back for him.
Question: What was with that series finale ending scene? Did Tony die or not? Did they do this to be open for interpretation?
Answer: The ending is purposely ambiguous and open to interpretation. The series creator once said: "There's more than one way of looking at the ending. That's all I'll say." He's also refused to outright explain what the ending meant.
Question: What is the name of the theme song of Roswell? I think it's something called "Here I am", or "I am what I am".
Answer: It's called "Here With Me", performed by Dido. It can be found on her album "No Angel".
Answer: Everybody including Angel saw her. After helping them and then being left alone to talk to Angel, Cordelia says that the Powers That Be let her come back one more time to help them. After Angel gets a call saying that she died, he realises that it was her spirit that was helping them the entire time.