Manually installing a Kernel/Init System

I would like to create a lightweight Linux distribution for my [Raspberry Pi 400](https://www.raspberrypi.com/products/raspberry-pi-400/) (for educational purposes). I have looked at [Buildroot](https://buildroot.org) and [Gentoo Linux](https://www.gentoo.org/) , but both require compiling OS components from scratch. How should I go about building a ROOTFS? I don't have much experience with bare-metal Linux... Ideally, I would: + Install the Kernel/[Firmware](https://github.com/raspberrypi/firmware) + Unpack the [Init System](https://github.com/OpenRC/openrc) + Get a [Package Manager](https://wiki.alpinelinux.org/wiki/Alpine_Package_Keeper) + Configure the system **Related:** https://unix.stackexchange.com/questions/87011/how-to-easily-build-your-own-linux-distro