Microsoft Edge has a built-in spellchecker powered by the Hunspell engine by default which is also used by LibreOffice, OpenOffice, Chrome, Opera, and others.
Microsoft Edge checks your spelling as you type in the browser. The spellcheck service is completed locally on the device; Microsoft Edge does not send info about your typing to Microsoft for spellcheck.
When you add a new language to Microsoft Edge, the browser downloads the dictionary for the new language to the device using HTTPS. The dictionary is used for the built-in spellcheck service. Deleting the language from your Microsoft Edge settings deletes the dictionary from the device. Guest mode does not use the custom dictionary from the profile or any added languages.
Your Check Spelling (spellcheck) languages will be listed in the same preference order you set in your preferred language settings.
If you add a word to the dictionary, it will no longer be highlighted (underlined in red) as a misspelled word.
If you mistakenly add a word to the dictionary, then you can remove the word from the dictionary to have it highlighted as a misspelled word again.
This tutorial will show you how to add or remove words you want for the spellcheck dictionary in the Chromium based Microsoft Edge.