Reinstall operating system by using Internet Recovery mode. When you are sure that the Mac is able to access Internet Recovery mode, there is another important task before installing the fresh operating system version. After a successful reinstallation process, the computer will contain only the default software (which is delivered with macOS). Reinstall your computer’s original version of macOS (including available updates): Press and hold Shift-Option-Command-R. Reinstall the version of macOS stored on your computer’s built-in recovery volume: Press and hold Command-R. In the Recovery app window, select Reinstall macOS Big Sur, then click Continue. Follow the onscreen instructions. If you’re getting ready to install macOS Catalina on your Mac, you have some preparations to make and a few choices to make. While many people just download the new operating system and let the installer do its thing, it’s a much wiser choice to do a bit of housecleaning and preparation before you perform the upgrade. The odd thing is that if you update, say, from High Sierra with a working LTE 4.0/Wi-Fi to Mojave, Continuity will work flawlessly after the update, but if you were to install Mojave from scratch, you would require CAT once again.
To answer your last question, use an Apple support site link to get the downloadable installers of choice.
These installers are just that. They only work to get the installer from Apple; then they install a macOS.
Some of these can take hours to build the system in your Mac; & may restart more than twice to do it.
Reinstall Catalina From Scratch
Within newer macOS, past High Sierra; these new upgrades may be called updates.
Have and use multiple backups, beyond & include Time Machine on external drives.
Look into and learn how to use Clone utilities; those may be useful to boot older OS.
No shady sites would have 'correct downloadable installers' from Apple, as you know.
Unless the undesired consequences of doctored content, should be found by accident.
How Do I Install Catalina From Scratch
Jul 25, 2021 3:24 PM