Just make them sneeze. Worked for me when I couldn’t get the button out of my nose. Yes, I was 5 ish. Because I thought it would fit. It did.

Trash that was thrown out of the window of a moving car. And yes, the authors did leave much ambiguity.

nope

No, the artifacts are not from when aliens previously lived on Earth. The artifacts are literal space trash that just happened to smack into our planet. Explanation straight from the source material. It’s a good book.