Depends what that means. It is more that a computer is either BIOS or UEFI, depending on its firmware (and potentially its firmware settings). An operating system installing itself under one or the other should then ideally make itself compatible.
If it helps, Ubuntu 20.04 on my computer (not a Librem laptop) has two partitions
- 512M EFI System partition, containing a fat32 file system, mounted on
/boot/efi - xG Linux filesystem partition, containing an ext4 file system, mounted on
/
defined in a gpt partition table.
Hence note that the partition table type (MBR traditional v. GPT) is an additional variable.
The disk also contains a protective MBR, with a single partition, covering the whole disk excluding the MBR itself and with partition type EE (GPT protective MBR). (The disk is not large enough to require any additional hackery.)