There are several ways to update or reinstall macOS. By far the most frequently used variant is to initiate the update via the system settings in the software update area. A new installation is also possible via the restore menu.
The different macOS versions could always be downloaded from the Apple website up to now, both the respective updates and the complete system package. But that could be over now.
Will Apple permanently remove downloads from macOS?
How recently discovered Apple no longer offers download of updates or installation packages for macOS Big Sur.
Such a download was not offered for macOS 11.0.1, the initial version of macOS Big Sur, or the most recently published update to macOS Big Sur 11.1. The downloads for macOS Catalina and previous versions can still be found on Apple’s website. These can be useful, for example, when several Macs are in use, so the large download packages do not have to be loaded repeatedly. It is not yet clear whether Apple has made the downloads disappear permanently. On the one hand, it would fit Apple’s product policy, which is not characterized by its openness and variety of options for consumers, on the other hand, there is at least the hope that this is a temporary phenomenon.