As the original A1200 hardware approaches its 35th anniversary, the physical ROM chips are degrading. The bits stored in those Mask ROMs are fading. Projects like the "Amiga ROM Replacement" (ARR) and "Kickstart Switchers" allow users to load Amiga-os-300-a1200.rom from modern flash memory.
Furthermore, the open-source "Aros" (Amiga Research Operating System) has created a replacement ROM, but for 100% compatibility with classic AGA games, nothing beats the original 3.0 binary. Amiga-os-300-a1200.rom
Without this specific ROM, an Amiga 1200 is a brick. It cannot read disks, initialize the blitter, or draw a single pixel. As the original A1200 hardware approaches its 35th
If you open a real Commodore A1200, you will not see a file; you will see a physical chip. Usually, it is a 40-pin DIP chip labeled with a sticker: To create the Amiga-os-300-a1200
To create the Amiga-os-300-a1200.rom file from real hardware, one would use a ROM dumper (like a TL866 programmer) to read the binary data from these chips and concatenate them into a single file.
If you burn Amiga-os-300-a1200.rom to a physical EPROM chip and install a large CF card, you will only see the first 4GB. You need a patch like TD64 or upgrade to Kickstart 3.1 ROM (physical chip) to fix this.