In my sibling comment about the overall systems aspects of the situation, I asserted that there was in fact enormously more information available for how to program in the 32-bit ARM assembly used by the ATSAMD20 than in Z80 assembly. This is an overview of that information, starting, as you did, from the Internet Archive's texts collection.

Searching the Archive instead for [arm thumb programming] I find https://archive.org/details/armassemblylangu0000muha https://archive.org/details/digitaldesigncom0000harr_f4w3 https://archive.org/details/armassemblyforem0000lewi https://archive.org/details/SCE-ARMref-Jul1996 (freely available!) https://archive.org/details/armassemblylangu0000hohl https://archive.org/details/armsystemarchite0000furb https://archive.org/details/learningcomputer0000upto https://archive.org/details/raspberrypiuserg0000upto_i5z7 etc.

But the Archive isn't the best place to look. The most compact guide to ARM assembly language I've found is chapter 2 of "Archimedes Operating System: A Dabhand Guide" https://www.pagetable.com/docs/Archimedes%20Operating%20Syst..., which is 13 pages, though it doesn't cover Thumb and more recently introduced instructions. Also worth mentioning is the VLSI Inc. datasheet for the ARM3/VL86C020 https://www.chiark.greenend.org.uk/~theom/riscos/docs/ARM3-d... sections 1 to 3 (pp. 1-3 (7/56) to 3-67 (45/56)), though it doesn't cover Thumb and also includes some stuff that's not true of more recent processors. These are basically reference material like the ARM architectural reference manual I linked above from the Archive; learning how to program the CPU from them would be a great challenge.

There's a lovely short tutorial at https://www.coranac.com/tonc/text/asm.htm as well (43 pages), and another at https://www.mikrocontroller.net/articles/ARM-ASM-Tutorial (109 pages). And https://azeria-labs.com/writing-arm-assembly-part-1/ et seq. is probably the most popular ARM tutorial. None of these is as well written as Raymond Chen's introductory Thumb material: https://devblogs.microsoft.com/oldnewthing/20210615-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210616-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210617-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210625-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210624-46/?p=10... https://devblogs.microsoft.com/oldnewthing/20210531-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210601-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210602-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210603-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210604-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210607-00/?p=10... https://devblogs.microsoft.com/oldnewthing/20210608-00/?p=10... (I'd link an index page but I couldn't find one.) Chen covers most of the pragmatics of using the Thumb instruction set well.

There's an ARM Thumb assembler in μLisp (which can itself run on embedded ARMs) at https://github.com/technoblogy/lisp-arm-assembler, which of course explains all the instruction encodings, documented at http://forum.ulisp.com/t/an-arm-assembler-written-in-lisp/12.... Lots of free software already runs on the chip, including FreeRTOS.

https://mcuoneclipse.com/2016/08/14/arm-cortex-m-interrupts-... covers the Cortex-M interrupt system, and lcamtuf has written an excellent tutorial for getting the related ATSAMS70J21 up and running https://lcamtuf.substack.com/p/mcu-land-part-3-baby-steps-wi....

Stack Overflow has 12641 questions tagged [arm] https://stackoverflow.com/questions/tagged/arm, as opposed to 197 for [z80]. Most of these are included in the Kiwix ZIM files of SO like https://download.kiwix.org/zim/stack_exchange/stackoverflow.... (see https://library.kiwix.org/?lang=eng&q=&category=stack_exchan...).