Reinstalling OS X

First, be sure to use Safari during upgrading/updating. When the spinning globe first appears, you will be asked to select your Wi-Fi network, then enter the network password. If you are connected to Ethernet, then disregard the previous sentence about Wi-Fi.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     This should install the original version of macOS that was pre-installed at the factory..

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) or Mac OS Extended, (Journaled.)
8. Provide a volume name, usually "Macintosh HD."
9. Click on the Apply button, then click on the Done button when it activates.
10. Quit Disk Utility and return to the Utility Menu.
11. Select Install OS X and click on the Continue button.

Upon successful completion of the installation, you can upgrade to Catalina: How to upgrade to macOS Catalina.

Well, you must have an Internet connection in order to install macOS. You can use Wi-Fi or Ethernet. I'd use Ethernet. If you use Wi-Fi, then you must select your network from the list, then enter the password for that network.