For the 90001 specifically, dumping can be done via:
The 90001 BIOS has zero support for the PlayStation 2 HDD (Hard Disk Drive) unit. While fat PS2s could run games off an internal hard drive, the 90001 BIOS removed the drivers for this hardware entirely because the chassis had no IDE ribbon connector. ps2 bios scph 90001
exploit, which uses the internal DVD player (often version 3.11U) to run homebrew from a burned disc without any hardware modifications. Key Hardware & BIOS Specifications BIOS Version Commonly found in (early) or Internal PSU For the 90001 specifically, dumping can be done
Use a modchip or an original Swap Magic disc to boot "uLaunchELF" from a USB drive, then run the "BIOS Dumper" tool to save the file to a USB stick. Key Hardware & BIOS Specifications BIOS Version Commonly
The SCPH-90001 was the last console Sony produced before they ceased production of the PS2 entirely in 2013.
If your 90001 has a Modbo 5.0 chip installed, the BIOS can be extracted via the chip's menu.