Raspberry Pi and its various competitors... the problem is you can't just go and buy ARM CPUs or boards if you're not either willing to upfront
serious amounts of cash or have personal relations to Qualcomm and friends (which is how RPi got their in).
You have a couple top tier vendors when it comes to actually performant SoCs:
- Apple, which exclusively produces for their own lineup, and has inarguably the best-performing chips by far in raw performance and power efficiency to a tune they're outperforming Intel
- Samsung, which use (almost) all their production for their own phones, tablets and TVs - I'm not aware of Samsung SoCs ending up in any third party product
- Annapurna Labs, which belong to Amazon for a few years now, and whose SoCs end up being used in servers (both AWS themselves and third parties, QNAP uses a fair few of their models)
- Qualcomm and Broadcom, who won't return your calls unless you're talking millions of dollars in volume, have all their documentation behind NDAs and have serious deviations from any kind of standards which is what makes supporting them in Android forks a huge pain in the ass, and even the RPi chipset lacks behind severely in something as basic as PCIe, see e.g. https://news.ycombinator.com/item?id=27150697
- Mediatek and Rockchip which are even worse than the above two regarding documentation/standards/BSP quality, but you seem to stand a chance acquiring one of their chips even if you're small volume - almost all el-cheapo/gongkai devices and RPi competitors are based on their chips
- NVIDIA with their Tegra lineup which seems to be used only on the Nintendo Switch, a few settop boxes and automotive
That's more than in the x86/64 ecosystem on paper, where the only players left in town are Intel and AMD... but these are based on some common standardized core around BIOS/UEFI/PCI, there is no such thing in the entire ARM space. If you want to design a device using ARM, you're left either relying on a more-or-less open BSP from the vendor or doing everything including the bootloader yourself, and it's left to each SoC/board vendor on how to wire stuff like sensors (yes, I'm aware of device trees, but they don't come close to the easiness of x86/64). In total, it's a fuckton of work for very low gain as the market share for desktop ARM (Apple aside) just isn't there.