Organizing and tagging plain text notes (macOS and iOS)
Try DEVONthink to Go for iOS, paired with one of the DEVONthink family of apps for macOS. It meets all of your criteria: it supports creating and organizing (with tags and hierarchical folders) notes in plain text, markdown, and other formats, as well as PDFs, images, and URL bookmarks. It also has a unique semantic artificial intelligence engine and other features.