Lets say you create a backlink on a page that isn't viewable to the public (you have to sign into an account or something to view). Can the search engines pick up on those backlinks even though it is private? If so, does it take longer? Thank you in advance!
No, search engine cannot read anything which is password protected. And, it is a good way of protecting unwanted crawling on your website.
Off course search engine crawl this type of link but it's part of black search engine can penalize this type of website.