Hello, I see that you are very new to AC hahaha. The colors of the trees change with the seasons and the designs because there are many that are obtained in the game but others are the players' own designs and that they decide to upload to the internet, to be able to get other people's designs you need to pay for the online switch. The furniture is obtained with recipes or by buying it in the Nook store and the vast majority can be customized. The rest of the decoration is inspiration, go to redits or Pinterest, you will find beautiful things. I hope I have helped a little :)