Why doesn't the BIOS see the Ubuntu partition?

Question

linux

I have a Biostar A770E3 motherboard and SSD, with Ubuntu Server 18.04.1 written on it.
The motherboard does not see the "ubuntu" partition on the SSD, while the SSD itself sees fine (displayed in the BIOS and when selecting a disk via F12). At the same time, ACER RS880PM-AM sees this section perfectly and loads ubuntu from it too.
A similar problem was a little more than six months ago, at that time it arose due to the fact that I burned the installation disk for windows 10, then installed it on the SSD on the same computer and only then inserted the SSD into Biostar. She decided to do this whole process on the Biostar motherboard. Now, for some reason, it does not help.

Answer 1

The BIOS does not see partitions, it only sees disks.
in UEFI, you can add items to the boot menu.
The BIOS works with the MBR, loads the program from the boot sector of the disk (grub for example), and that in turn shows the boot menu.
UEFI works with GPT, menu items are stored in nvram, or the default bootloader is loaded.
CSM is used for BIOS compatibility.
What format is the disk in, is CSM enabled, what boot options are used (uefi only, legacy only, uefi&legacy)?