I believe it uses your selected provider. I looked in about:config and saw no URLs when searching for things like "search", "Suggestions" etc.

really the only way to be sure is to fire up wireshark. But I am on a work computer with a TON of crap running that is quite chatty. So i didnt look to verify.

But even on my browser I see this: https://imgur.com/a/odiHIzy

That said, my search provider is Kagi and I also dont see my search quota increment even with search suggestions enabled....

I was under the impression that Firefox uses your selected search provider for this feature.