I’m not sure about other fields, but in computer science (or more specifically AI/ML) conference papers are completely free of any “professional” editors — burden is entirely on authors to handle their own editing. But I have noticed 0 disparity between conference and journal papers in terms of copy-editing quality. And my one experience of submitting to a journal matches other descriptions — the “professional” editors introduced spelling, grammatical, and even factual errors that weren’t there in the first place. (Most egregiously, they edited a grad student’s bio to award him a PhD that he didn’t have yet. WTF.)