Avatar the last airbender?

Well Aang was the last airbender. And the next avatar will be a waterbender due to the cycle. And plus origional airbenders were sky bisons. And appa is the last one. So how would the next avatar learn how to airbend? Because it's in the story for the avatar to learn how to bend all four elements. And also benders learn how to bend from other things. Moon,Badgers,Dragons,bisons, but how did people learn from them? Well in that case couldn't anyone be a bender? Or in another case coulodn't any one learn all four elements? (even though only the "Avatar" can bend more than one.) Please if u answer please answer all my questions.