The instruction set reference (the architecture manual, to give it its proper title) has been freely available from ARM for some time, though you needed to create a website account and agree to a clickthrough license. It's more recently been made available as a simple download: https://developer.arm.com/products/architecture/a-profile/do... for cortex-a profile https://developer.arm.com/products/architecture/m-profile/do... for cortex-m profile

For documentation of the devices in a particular SoC you'll need the reference manual from the SoC manufacturer -- they of course vary in how easy it is to find those docs.