Two Free Apps for Mac OS Installation Ease
On most modern Macs, the later Intel builds and all Apple Silicon models, you can boot into recovery mode, access disk utility and download and install a new operating system. Unless you can't. Then you have a problem. Or, maybe you are experimenting with Open Core Legacy Patcher to install a later version of macOS on a device that doesn't officially support it. In that case, you are going to need a copy of an OS, preferably bootable, and some sort of media to copy that OS installation onto. You can do the installation through other methods, like target disk mode...