You dont need 3rd Party plug-ins until you understand what Logic does, and what it has included as stock.
Yes you would purchase on disk or download a new plug-in.
After fully installing it, you would open up Logic.
Go to the track, and open up the plug-in on that track.
But again, at this point you will only be wasting money. Logic has great plug-ins, possibly some of the best ones in stock DAW's.
Learn to use them. You'll be able to complete a whole project with only it. In fact you should challenge yourself to complete 5 songs, start to finish including the mix, using only Logic's tools. Without doing so, you have no way to compare and understand the difference between plug-ins and you'll run around buying everything new, because you overlooked the fact, that you already have that exact thing in your tool box already.
For additional sounds you can adjust the presets in Logic so that they fit what you want to sound like. This is true for both instruments/samples and effects.