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If I reinstall macOS using Command (⌘)-R during startup, will I have to reinstall all the apps that aren't included in the OS itself? The info page says "Reinstalling macOS does not remove data from your Mac." Does that mean documents, preferences, passwords, and so on will be intact after the reinstall?
iMac
In modern versions of macOS, reinstalling macOS from Recovery simply "replaces the Core Operating System Files that allow macOS to startup correctly" directly from Apple's servers. This way, if such a file was damaged or attempted to be updated during a software update but didn't take exactly, the reinstall would pave over that with the latest version of macOS your hardware supports (and healthy versions of said files).
The only time your data is destroyed is if you erase your hard drive with disk utility before reinstalling macOS (on purpose), or if attempting a reinstall, your hard drive that holds your data failed (and as a result your reinstall failed), or if reinstalling caused your user account to be unexpectedly unlinked and or deleted despite the technology not intending to do so.
Avoiding data loss usually is as simple as doing a Time Machine backup first if possible. However, it's understood that most people would wish to consider reinstalling to resolve a startup issue (making doing a backup impossible).
Either way, you could consider taking it to a local Data Recovery specialist in your area to see about withdrawing files first if you'd rather not take any risks.
TL;DR - reinstalling doesn't delete anything. It just replaces/repairs the Core files required for the macOS to start up correctly again or whichever apps are included in macOS by default that you cannot delete (EG Safari, Mail, etc).
In modern versions of macOS, reinstalling macOS from Recovery simply "replaces the Core Operating System Files that allow macOS to startup correctly" directly from Apple's servers. This way, if such a file was damaged or attempted to be updated during a software update but didn't take exactly, the reinstall would pave over that with the latest version of macOS your hardware supports (and healthy versions of said files).
The only time your data is destroyed is if you erase your hard drive with disk utility before reinstalling macOS (on purpose), or if attempting a reinstall, your hard drive that holds your data failed (and as a result your reinstall failed), or if reinstalling caused your user account to be unexpectedly unlinked and or deleted despite the technology not intending to do so.
Avoiding data loss usually is as simple as doing a Time Machine backup first if possible. However, it's understood that most people would wish to consider reinstalling to resolve a startup issue (making doing a backup impossible).
Either way, you could consider taking it to a local Data Recovery specialist in your area to see about withdrawing files first if you'd rather not take any risks.
TL;DR - reinstalling doesn't delete anything. It just replaces/repairs the Core files required for the macOS to start up correctly again or whichever apps are included in macOS by default that you cannot delete (EG Safari, Mail, etc).
Why do you want to or think that you need to reinstall macOS?
First try Safe Mode followed by a normal restart to see if that clears up your issue(s).
see > How to use safe mode on your Mac - Apple Support
Then if you are still having issues, download and run EterCheck Pro > https://etrecheck.com on your iMac and post back here with your EtreCheck Pro report as per > How to use the Add Text Feature When Post… - Apple Community
In Theory, doing an In-place installation replaces missing or corrupted Elements of the Operating System without touching Personal Data, Preferences and already installed Third Party Applications.
Good practices would to perform a Time Machine Backup before attempting
The "why" is because I have an issue where my Mac decides to restart itself when it's left to its own devices for a while (in situations where a properly running OS would go to sleep). I thought a normal OS update might solve the problem, but no. I've tried resetting the PRAM and such, but nothing has worked, so I've been considering this more radical step.
I keep regular backups so I'm not worried on that front. I just don't want to have to reinstall a bunch of apps and so on.
Reinstall macOS using Command (⌘)-R during startup
