"Sriracha" couldn't be trademarked because it is the name of the sauce. It's like someone trying to trademark "Kansas City style Barbeque Sauce" -- can't be done.
My (limited) knowledge is solely of US law, but different companies with the same trademark can coexist in different industries, e.g. Delta (the airline) and Delta (the faucet company).
Yes. In the US, if Apple sold apples, its name would be classified as descriptive and they would not have any exclusive right to that mark when selling apples. Other companies could also label their apples with "apple" and Apple could not stop them.