Question

let's Go wait outside until the library opens is it a statement

Answer 1

No, this is not a statement, this is a suggestion. a statement simply just states something, like "Apples are delicious". Yet this is more of a suggestion, saying "Let us, go outside and wait, statements usually don't yield actions

Answer 2

No because it’s someone suggesting that they should go wait outside the library a statement would be like “we waited outside the library.”