I think all the confusions in this post come from coexistence of old and new technologies and abuse of terminology such as mentioned in https://en.wikipedia.org/wiki/System_partition_and_boot_partition. I am not sure if creators of computer technologies or terms already try to avoid making things that are confusing and make things clearly and coherently documented. I can't figure them out on my own.
https://en.wikipedia.org/wiki/BIOS_boot_partition seems to mention different scenarios for different combinations of boot sector (GPT or MBR), firmware (BIOS or UEFI), bootloader (GRUB or ...?). I feel lost in reading it:
A BIOS boot partition is needed on GPT-partitioned storage devices to hold the second stages of GRUB. On traditional MBR-partitioned devices, the disk sectors immediately following the first are usually unused, as the partitioning scheme does not designate them for any special purpose and partitioning tools avoid them for alignment purposes. On GPT-based devices, the sectors hold the actual partition table, necessitating the use of an extra partition. On MBR-partitioned disks, boot loaders are usually implemented so the portion of their code stored within the MBR, which cannot hold more than 512 bytes, operates as a first stage that serves primarily to load a more sophisticated second stage, which is, for example, capable of reading and loading an operating system kernel from a file system.
There is also an article for EFI partition https://en.wikipedia.org/wiki/EFI_system_partition. Since UEFI replaces BIOS, and BIOS is stored in ROM not in any partition, I thought UEFI is stored in ROM too, but fell into confusion when seeing "EFI partition". Is "EFI partition" a partition dedicated to store UEFI, or some bootloader program?
I also found https://askubuntu.com/a/579259/1471 which seems to explain better, but am still lost in the wording "booting Linux in legacy mode" and "booting another legacy operating system" and more.
Can you try to use plain language to describe what scenarios there are?
What are being deprecated and what are being adopted more and more and therefore worth my time to know?
Here is what I heard
Some OS books, such as Modern Operating System, say that part of a boot loader is stored in the MBR sector of a boot disk and and the rest of it is stored in the boot sector of a boot partition. There doesn't seem to be a partition dedicate to firmware or bootloader. I understand that a boot partition is where OS is installed, not a partition dedecated to store (part of) a bootloader. The MBR sector of a boot disk is also not a partition dedicated to store (part of) a bootloader.
I have an old laptop, with GPT boot disk and a BIOS GRUB partition. I don't know if that partition is dedicate to store the full program of GRUB, or just part of GRUB, and the rest of GRUB is stored in the boot sector of the partition where the OS is installed, or ...?
$ sudo parted -l [sudo] password for t: Model: ATA ST1000LM014-1EJ1 (scsi) Disk /dev/sda: 1000GB Sector size (logical/physical): 512B/4096B Partition Table: gpt Disk Flags: Number Start End Size File system Name Flags 1 1049kB 2097kB 1049kB bios_grub 2 2097kB 16.0GB 16.0GB linux-swap(v1) 4 116GB 716GB 600GB ext4 3 716GB 1000GB 284GB ext4
I found an old but newer laptop, with GPT boot disk and a UEFI partition. I have similar questions as in 2, and I don't know if the UEFI partition stores UEFI or some bootloader (partial or full).
$ sudo parted -l [sudo] password for t: Model: ATA TOSHIBA MQ01ABF0 (scsi) Disk /dev/sda: 500GB Sector size (logical/physical): 512B/512B Partition Table: gpt Disk Flags: Number Start End Size File system Name Flags 1 1049kB 538MB 537MB fat32 EFI System Partition boot, esp 2 538MB 500GB 500GB ext4