Interceptor11 Saif:
"I have a MacBook, I wanted to install Windows and cleaned the disk, but I accidentally deleted everything. I had version 10.15.7 What should I do?"
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Install the macOS, First...
A. Restore From a Time Machine Backup:
Did you ever create a Create a Time Machine Backup of your Mac with all of this installed? If so, restore this data from it, using this link: Restore items Backed Up with Time Machine on Mac - Apple Support. If not, then take this as learning lesson, knowing to create a back every-so-often. That way, you can have something to get your info back, should anything go wrong from then on out.
No Backup?...
B. Reinstall the macOS Anew:
Use my User Tip: How to Reinstall the macOS Anew - No TMB Prior to Upgrade - User Tip. Format the hard drive with the applicable macOS, on step D.
In Depth:
Pretty much, it is a matter of:
- Erasing: and formatting your hard drive to APFS
- Installing: [the applicable macOS] via a USB Bootable Installer
- Restoring: info from a Time Machine Backup, using Migration Assistant