Programming

I personally started learning microcontrollers using an Arduino dev kit, and then progressed towards compiling the code myself using GCC and loading it directly to the Atmel 328p (the microcontroller from the original Arduino dev kits).

But nowadays, I would recommend the MSP430 dev kit (which has excellent documentation for its peripherals) or the STM32 dev kit (because it uses the ARM32 architecture, which is very popular in the embedded hardware industry, so would look good on your resume).

Regarding userspace drivers, because these are outside of the kernel, such drivers are not kept in the repositories for the kernel. You won't find any userspace drivers in the Linux or FreeBSD repos. Instead, such drivers are kept in their own repos, maintained separately, and often does unusual things that the kernel folks don't want to maintain, until there is enough interest. For example, if you've developed an unproven VPN tunnel similar to Wireguard, you might face resistance to getting that into the Linux kernel. But you could write a userspace driver that implements your VPN tunnel, and others can use that driver without changing their kernel. If it gets popular enough, other developers might put the effort into getting it reimplemented as a mainline kernel driver.

For userspace driver development, a VM running the specific OS is fine. For kernel driver development, I prefer to run the OS within QEMU, since that allows me to attach a debugger to the VM's "hardware", letting me do things like adding breakpoints wirhin my kernel driver.