To be honest, I really don't know a good KiCad tutorial. I kind of just banged myself against it until I learned. But, I had previous experience with Eagle, Altium, and Cadence Allegro.
Das U-Boot is the embedded bootloader and is pretty standard for embedded ARM systems (and others! https://github.com/u-boot/u-boot/tree/master/arch).
On a dev-board prototype before, I had also used Buildroot (https://buildroot.org) to create the minimal root filesystem image. This time around, I decided to use Debian since it makes installing packages I want much easier. For example, I can just apt-get install the USB wifi firmware packages instead of hunting them down and including in a manually generated image. It's still using a custom compiled kernel though, since I have custom drivers for things like the graphics hardware.
I'm trying to use Rust for as much as possible (I like Rust a lot). The STM32L0 runs rust, I bodged together a framebuffer driver in Rust, and the games in userspace are also in Rust. I'll post about the Rust framebuffer driver at some point.