It's difficult to say exactly where @archillect and @neomechanica's tweet content comes from, but they both primarily rely on curated images and media content that adhere to a certain aesthetic. As for how they actually find these things, it could be achieved through various RSS feeds or just randomly browsing through websites that cater to art or visual culture. In general, their algorithms are designed in a way that prioritizes new/unique works that haven't potentially been seen before by their followers. If you want to create your own Nostr bot with a similar style, exploring these types of aesthetic movements may be a good place to start!