Here our guide to reinstall the System Software on the Dell Optiplex 9020 SFF with the including specs.
Before You Begin
1 x Dell Optiplex 9020 SFF
1 x 32GB USB Flash Drive (for macOS)
1 x 32GB USB Flash Drive (for Windows)
1 x 8GB USB Flash Drive (for bootloader and post installation)
1 x USB WiFi Adapter
1 x Bluetooth Adapter
1 x Bluetooth or Wired Keyboard
1 x Bluetooth or Wired Mouse
1 x Smartphone or Tablet with Internet
1 x HDD/SDD or Second PC with Internet
1 x USB to SATA Enclosure
Place your HDD/SSD drive into a SATA to USB Enclosure.
Connect to an iMac/MacBook and fully install a macOS version that both devices can support.
Restart to the internal drive of the iMac/MacBook.
Download the Bootloader and place in the (ESP) EFI System Partition.
Format the 1 x 8GB USB stick and copy the Bootloader to the ESP.
Download any Utility Software needed for post installation to the 1 x 8GB USB stick.
Eject the HDD/SSD drive and add it to the Dell Optiplex 9020 after it is configured.
Keep the 1 x 8GB USB stick for emergencies to boot the system.