I do agree with Glenn.
Here, though, the answer is simple : yes.
If you struggle to find where X-plane has been installed by Steam, open Steam, right click on X-plane 11 and choose "browse files" or similar.
A downloaded plane has to be unzipped and placed in "X-plane 11/aircraft" folder.
It is very important to read all included instructions because some aircraft might require more steps.
One last thing : for some aircraft, on X-plane aircraft selection window, you need to check the box "show aircraft from previous versions" in order to see them in the list.