I thought arm was a lot more open than x86 based tech?
x86 has its roots in the PC, which was an open and relatively standardised platform.
ARM's rise to popularity came later, and there are literally thousands if not tens of thousands of different SoCs with an ARM core, which have nothing in common but the core. All the locked-down mobile devices are in this category, and their SoC manufacturers have been extremely reluctant to release much in the way of documentation.
x86 was and is proprietary (ARM's ISAs and designs are as well). The wider x86 platform is somewhat standardized, but that doesn't mean it's open any more than Apple computers using Thunderbolt or PCI or USB or DDR standards mean they are open.
https://www.newegg.com/acer-cp513-1h-s60f/p/2S3-0008-003J5?I...
https://www.newegg.com/p/1HD-005N-00048
https://www.newegg.com/nvidia-945-13541-0000-000/p/N82E16813...
Etc.
> Do they sell ARM processors and motherboards?
Yes some of those boards are essentially motherboards with ARM processors on them.
> Are there even standard sockets for them?
Not that I know of for ARM chips. If by "standard" you mean Intel-proprietary or AMD-proprietary, then there is be no reason that an ARM vendor or ARM Ltd. itself could not define a socket specification. That's not a matter of openness or proprietariness of the platform.
The answer: it isn't open, it's proprietary. Check out x86 history, it's kinda interesting.
x86-64 is patented (and the earliest CPUs with it are now over 20 years old, so probably expiring soon if not already), but original 32-bit x86 is long out of patent and there are/were quite a few other manufacturers.
https://en.wikipedia.org/wiki/List_of_x86_manufacturers
https://en.wikipedia.org/wiki/List_of_former_IA-32_compatibl...