For those using an iPhone, iOS 18 general release is scheduled for 1:00 PM eastern time today. If your iPhone runs iOS 17, it will run iOS 18. I’ve been running betas for a couple of months and now the release candidate for the week it’s been available.
Barring significant bugs discovered (there are none of which I’m aware), the release candidate is what gets generally released. For me, the release candidate is stable and what I see online suggests the same is, generally, true for others as well.
In addition to iOS 18, macOS Sequoia, iPadOS 18, watchOS 11, tvOS 18 and visionOS 2 (I think) launch today as well.