How is Basic Spell Check working

Hi stesch79 

Well, that's an interesting question. Actually it's a bit of both.
For the languages installed on your Windows OS in "Settings => Time and Language", Edge and even native Chrome use built-in spell check of Windows, whereas for the additional languages chosen in Edge settings, the dictionaries are still the Hunspell local/offline files, like: en-US-10-1.bdic, en-GB-10-1.bdic ..

They get updated with each restart of edge (if there's a newer version), and can be found in each user's Edge User Data folder
(e.g. C:\Users\<UserName>\AppData\Local\Microsoft\Edge\User Data).  

So, Edge/Chrome spell check is indeed kind of a chimera, at least on Windows based systems 🙂

Little side notes: All the above only applies to the "Basic" spell check. If you've activated "Microsoft Editor" in edge://settings/languages then a Microsoft online service is used to process all spell checking!

The use of Window's built-in spell check for system languages is a joint development by Microsoft and Chromium project simply called "Windows Spellcheck" and was first rolled-out in 2020.