Each paper you write, download, order, etc. can be checked with special tools. Plagiarism checkers nowadays are available practically everywhere. You can use them online and offline, in a web browser or download an application to your laptop. There are also free versions along with the ones you’d have to pay for. As for me, there is practically no difference between them. The main thing here is your convenience. If you feel uncertain about the application you try, you can always find several other resources, to double or triple check.
As far as I know, the most popular plagiarism checker in the U.S.A. right now is Turnitin (turnitin.com). Still, there are many more. For example, Grammarly app has an inbuilt plagiarism checker. Or there is a nice resource online, called Unicheck (unicheck.com). You can use one of them, or all three at once to run the plagiarism check of your work.
If you worry about the paper you download from the StudentShare database; you should remember one thing. Those are samples, kindly uploaded by students who wrote them and submitted them during their studies. Thus, if you submit it as own work, it would be 100% plagiarism. You may only use it as an example of work. You can develop the ideas found there, use the outline to structure your work or use the bibliography for own research. Also, Studentshare has two great options to make a paper you download unique, namely
Hide a Paper and Rewrite - thus you’ll get the unique paper on the issue done for you by writers, but cheaper, than ordering it from scratch.
Still, if you order the writing service StudentShare offers, you can be sure the paper would be 100% original. You are also welcome to order plagiarism report, to be sure of that. Also, you can always check it yourself with the resources you like.