When I boot Ubuntu from a USB key, I do access the Live Ubuntu but Ubuntu can't see the internal Mac mini 2018 disk even though it detects my extra external hard drive. So, it is impossible so far for me to install Ubuntu as a standalone OS. I already remove the T2 chip security and the SIP security (not sure what it is). Do I have to trash my Mac mini on an Apple store bay (it is quite tough ...) or to burn it ? Or some voodoo ?
The USB installer doesn't recognize the disk because Ubuntu can't read the APFS filesystem. Your only options are to use recovery mode to erase the filesystem from the disk so that Ubuntu can read it or install Ubuntu on a USB drive and build the apfs-fuse driver from source (you can get it via github) so that Ubuntu can read and format the disk. I recommend the first option as it's far easier.
After that, your Ubuntu installer will see the disk and you can proceed.
You can't see the internal NVMe because the linux kernel requires custom patch for T2-chip based macs. This patch is originally for MBP 2016+ but it must work fine with the Macmini 2018.
PS. It has nothing to do with APFS.