* Apache Nutch - So they're crawling either some part of the root itself or some other websites to find subdomains. Honestly might help to query CommonCrawl too.
* Calidog's Certstream - As you said, you can look at the CT logs
* OpenAI Embeddings - So I guess it also uses LLM to try to generate ones to test too.
* Proprietary Tools - your guess is as good as mine
Probably a common list of subdomains to test against too.
Seems like multiple techniques to try to squeeze out as much info as possible.