Do Web Browser Built-In Spell Checks Use Local Dictionary?

Question

I am curious if enabling the spell check in Firefox/Brave/Chrome/Opera sends text to an outside source to be analyzed or if the dictionary and computation is done on my machine.

Piskvorrr · Accepted Answer

That would have been a better question on SE; alas. That said, yes, the spell checks use a local dictionary, at least on FF. There seem to be various extensions that can check remotely, but those are definitely not enabled, or even installed by default.