Patent examiners can look basically anywhere a member of the public can and some other places.
The internal search tools check a lot of patent databases and are quite good, but take time to learn.
The USPTO has a lot of subscription databases as well, including fancy AI/ML-based ones.
Many examiners will also search normal search engines like Google, though this can be tricky for legal reasons. If the application was not published yet then examiners are not allowed to get very specific in the search and other search engines as that could release confidential information to the search engine. The USPTO has agreements with the subscription databases to keep the searches confidential but no such agreement exists with Google.
In my view making new search tools like https://www.priorartarchive.org/ would not help the situation too much. It would be better to integrate more databases into the existing USPTO tools, as they are designed for serious power-searchers, and would make the new databases more visible. The internal search tool is much faster than the alternatives and operates by keyboard. Point-and-click search is much slower by its nature. Speed really is critical when time constrained and I think that is something not appreciated outside of patent organizations.