Done some more homework... It seems a opencore bug/incompatibility with (at least) this specific NVME Samsung ssd.
I installed another SSD in the bottom slot (it's another Samsung [PM871 SATA not NVME]).
I copied the EFI folder to the EFI partition and it worked (I even booted from NVME Big Sur partition....)
Take a look at enclosed screenshots.
View attachment 503401View attachment 503389
Time for an opencore bug report ---
edit: Tried to mount the NVME ssd in bottom slot, same problem.
windows and mint are bootable (opencore option in UEFI setup or F7 menu is not appearing).
I believe it's a generic Opencore issue with some NVME disks.
About your question:
when I boot Opencore from USB, or SATA, I see only windows as alien OS (I still have not configured Linux as "CustomOS") BUT from the F7 menu I can choose the other refi boot loader installed in the same EFI partition.
Therefore is not a matter of malformed EFI folder structure but a plain OpenCore issue.