I am writing a systematic review based on the prisma guidelines, I used certain databases to find the appropriate papers to include in my review. I need to cite these. How do I cite these database engines?
You don't have to cite them, you can just list them. You can see examples in the Cochrane library which I think is all open access. Here's one example https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD009747.pub2/full