Certainly a Noob/Dumb question

Hello i am really REALLY new in creating 2D game with Unity and i have a questions about Assets image and creating sprite from them.

When in a assets image you have tiles that take 32*32 pixel so you slice your image to create a tiles set with 32*32 tiles BUT in the same image you have for example a tree that take multiple 32*32 tiles to be drawn.

How do you create a unique tree sprite from the asset png ? Do you copy the asset image and slice only the tree ?

As you can see i only begin to use Unity, and i pretty sure the solution will be something dumb cause every tileset i saw have tiles and object with different sizes and shapes so there must be something i don't understand.

(french here so sorry for english mistakes) Thank you in advance for taking time to answer this nooby question.