You connect your X account. Sift reads the posts you have bookmarked, stores them, gives each one a category, a few tags and a one-line summary, and makes the result searchable. It emails you a digest of what you have saved, daily or weekly by your own choice, and you can export the library as a CSV file.
What it is not: a backup of your X account, a scraper, a way to reach content you cannot already see, or a substitute for X. It reads what X's API returns for your own bookmarks, and nothing else.