This is a search across three photography platforms rather than a library of its own. One query is sent out to Unsplash, Pexels and Pixabay and the results come back together, which saves opening three sites and running the same search three times when you are hunting for a cover image.
Because the pictures belong to those platforms and to the photographers who uploaded them, the terms that govern your use are theirs and not ours. Each platform publishes its own licence, each revises it from time to time, and the details differ in ways that matter — particularly around recognisable people, private property, logos and trademarks that happen to appear in a shot.
Search results arrive twenty at a time and you can page through for more. You can mark images as favourites while you compare candidates; those favourites are kept in your own browser, so they survive a page reload but not a cleared cache, and they do not follow you to another device.
From Unsplash, Pexels and Pixabay. The search runs against those three platforms and shows their results together; the images are hosted by them and belong to the photographers who uploaded them. Each result identifies its source so you can go back to the original listing.
That depends on the licence the source platform attaches to that image, and on what is depicted in it. All three publish permissive terms for most content, and all three have conditions and exclusions that change over time. Check the licence on the image's own page rather than relying on a general statement anywhere else, including here.
Attribution requirements are set by the platform and vary between them. Even where credit is not required it is often appreciated and costs you a line on the copyright page. If you are unsure, crediting is never the choice that creates a problem.