When submitting your text into a plagiarism checker; it will exist on a database so is it possible that you could be done for plagiarism by this also?
Student life, in partnership with UEA
they look for phrases similar or identical to other texts that lack citations and generate a score based on the occurrence of similarities
a tutor wouldn't look twice at a score of like 20 (out of a 100), it's only once your essay gets to like 50 that the marker might call you in and ask what happened
especially at undergraduate level, it's highly unlikely you'll come up with a truly original idea because what you're doing has been done by thousands of people before you - many of your essays will be literature reviews
That depends on the subject and context. I just took a critical "tutors eye view" at a round of lab report submissions and anything over 5% got reviewed in more detail. Turnitin flags up single lines of text or more with clickable links to potential sources for a reviewer. 20% can be fine if its lots of little snippets that are just common phrases, or are well identified and references quotes or book / paper titles in the Bibliography. 2% of gratuitous cut n' paste from an unreferenced source all in one paragraph can be deeply problematic.