I'm the developer of OneLook and happy to answer any questions about it! It's been running continuously since 1996 and I've been (very slowly) adding features to it over the past couple of decades.
OneLook crawls and indexes the headwords from ~1000 online dictionaries and glossaries. When you do a wildcard search, it searches all of these lists at once. Clicking on one of the results will show you the sources. The sites range from small subject-specific glossaries with a few dozen words, to commercial dictionaries like Merriam-Webster and OED.