StableReleaseUpdates will no longer appear in update-manager at the same time for all machines. Instead a subset of machines will be selected at random to receive the update first. The update will only be made available to everyone if there are no serious regressions encountered by the first set of users. There is still a testing process completed by Ubuntu developers before any users receive the update.
Source: PhasedUpdates and Brian’s Blog.
Prior to Ubuntu 21.04, if you update via the terminal, you bypass the phased update system and will get all updates as soon as they're released. Read the comments in Brian's Blog for how to opt out of phased updates if you wish.