More in-depth and up-to-date info at this page: IBM PS/1 model 2011

At boot you are presented with a mouse driven graphical interface, loaded directly from ROM:

(U.S. version shown)

A ROM drive is mounted as D:\ containing the IBM DOS 4.01 files.

The DOS, BIOS, and GUI are stored in 27C010 1M EPROM chips.
The ROM subsystem consists of either two 128K-by-8-bit modules (U.S. models) or four 128K-by-8-bit modules (other countries) arranged in 16 bit words. 512KB of ROM are addressed from F80000 to FFFFFF. Addresses FE0000 to FFFFFF are also addressed as 0E0000 to 0FFFFF.

Here you can download the 4 EPROMs images of an italian PS/1  model 2011  (the ones in the image above):

u18-x1-1057630.bin
u23-x2-1057757.bin
u28-x3-1057759.bin
u36-x4-1057628.bin

In order to read the content, you need to merge U36 with U18 and U23 with U28, taking one byte from the first bin file, one from the second, and so on and so forth, actually interleaving the data.
Or you can download the already merged versions:

u23-u28.bin
u36-u18.bin

u23-u28.bin contains the system BIOS, which I already extracted for your convenience:

SYS_ROM.BIN

Like all ATs, the BIOS is mapped in memory at F000:0000 and is 64KB in size. The BIOS includes the BASIC interpreter.

Every detail about the hardware can be found inside the IBM’s book Technical Reference for PS/1 Computer (P/N 57F1970).

This book is rarer than a pink unicorn, but I’ve managed to get hold of a copy.

Complete book (532 pages), 1st edition 1990
searchable and indexed (there’s some mistake in the OCRred text, notably some 0 -zero- mistaken for O or C, 3 for B, etc, so consider this when searching for text)

Technical Reference for PS1 Computer – PN 57F1970.pdf

Single sections
every file is searchable

Preface.pdf
TOC.pdf
Section 1. System Description.pdf
Section 2. System Board.pdf
Section 3. System Board IO Controllers.pdf
Section 4. Power Supply.pdf
Section 5. Keyboard.pdf
Section 6. Mouse.pdf
Section 7. Displays.pdf
Section 8. Drives.pdf
Section 9. Modem RS-232C Interface Cards.pdf
Section 10. 80286 Instruction Set.pdf
Section 11. Characters and Keystrokes.pdf
Section 12. Compatibility.pdf
Appendix. Options.pdf