extracting it will take the files out of the .rar and place them someplace else, in most cases in the same directory as the zip folder inside a folder with the same name of the zip, unless you tell it otherwise that should be the default setting,
but its not that simple you cant just download a model and add it to an existing one in a simple lil step, read some of the modelling tutorials in the stickys here if you want to learn how, you need to decompile both models then merge them then recompile to add the other part to it,
/closeded