You can (well, could) put in any live URL there and IA would take a snapshot of the current page on your request. They also actively crawl the web and take new snapshots on their own. All of that counts as 'writing' to the database.
Yep, but I didn't mention that because it's not a part of the "Wayback Machine", it's just the general "Internet Archive" business of archiving media, which is for now still completely unavailable. (I've uploaded dozens of public-domain books there myself, and I'm really missing it...)