i solved all the puzzles and opened the door but there is still one last puzzle on the puzzles' screen ( the last one on the bottom left) i can't figure it how to solve it , can someone help me please?

The solution is in the Official LDW VV3 Guide.

OK thanks , i solved it
i just have 2 little questions now !
-now i have all the puzzles solved , so what should i do now ?
-how can i help villagers live till 90 years old (for the reward) ?

You can finish collections and get the awards.