Reinstalling OSX (or Ubuntu) on a entirely blank mac mini 2009

Solution 1:

Your question do not entirely explain how you originally installed Ubuntu. The latest versions of Ubuntu can be installed on Macs to be EFI booted using the Mac Startup Manager. However, rEFInd and also be used to EFI or BIOS boot Ubuntu.

Your question does not address why you can not use the Ubuntu USB installer prepared using the instructions posted here or here.

Based on the information posted in your question, I am not entirely sure you can install Snow Leopard 10.6. The procedure for acquiring and creating a USB bootable Snow Leopard installer is given here. You may also try burning the ISO to a DVD.

You could also try installing El Capitan from USB. Apple's instructions for downloading and creating the USB installer are given here. Many alternate methods are given here.

Response to OP's ADD ON

You should be able to use the current version of Etcher on your MBP running Big Sur to create a USB EFI bootable Ubuntu 20.04 Installer. You can use this USB installer to try to install an EFI booting Ubuntu on your 2009 Mac mini. I say try because I do not believe the method used to install the previous Ubuntu has been established. Occasionally, a Mac can be found that can not EFI boot certain versions of Linux. You can also try installing Ubuntu to BIOS boot on your 2009 Mac mini, however the installation procedure is complicated by the inability of the firmware on your Mac mini to BIOS boot from USB.