Bootlin, an embedded Linux consultancy formerly Free Electrons, publishes kernel contributions, free training materials, and tools for embedded systems.
Commenters recommend its free embedded Linux and kernel training courses and slides as a strong introduction to boot setup and kernel building.
Will you be doing embedded Linux? Embedded RTOS? Bare metal? Microcontrollers? SoC (say, FPGA with a hard processor core)?
You can do a lot with QEMU. https://bootlin.com/ has a lot of great, free training material.
https://bootlin.com/doc/training/embedded-linux-qemu/embedded-linux-qemu-labs.pdf
is one of my favorites.
Learning to cross-compile, do embedded debugging, the process of booting an embedded system (which varies depending on the answers to the above questions), learning how to read a technical reference for the processor you’re using as well as for peripherals you’re likely to interact with - SPI, i2c, UART, maybe PCIe, are all handy skills. Learn a bit about JTAG, hardware, reading schematics, etc. Even being able solder is helpful.
There may be books (I had a great embedded Linux book when I started) but there are lots of online materials too. Check out https://platformio.org/
There are fun embedded boards and projects for microcontrollers too - micropython on an rpi pico, tinygo, eLua, etc.
Why reinvent the wheels over and over again?
Just port coreboot, or something similar to the likes of https://cwwk.net/ which have done the hardware part sufficiently.
Or even better, organize to have some group/org paying them to port it to their HW.
Or/and get https://bootlin.com / https://www.collabora.com involved,
if they are disinterested, have them coordinate the effort.
They should know how to do that, instead of wasting time with reverse engineering all sorts of crap.
Apply same due dilligence to the firmware running on the NIC/switch.
Then continue with the 'optics' in the SFP+.
They are 'smart' nowadays, and often run Linux, or some RTOS, too.
See https://pon.wiki/xgs-pon/ont/bfw-solutions/was-110/#boot-log for just one example.
Diz iz dä äyge of phybre.
Äye vanna häff lain speed! Arr!
I don't know that I'd recommend learning FreeRTOS for fun because you should basically never be using a traditional RTOS if you can possibly avoid it. Unless you really like reinventing stuff that you get "for free" on Linux, there are remarkably few advantages beyond certifiability (which FreeRTOS in particular doesn't get you), easier porting (just buy something that actually runs Linux) and lower resource use (more work to actually achieve!).
As for Linux, your first stop should be the bootlin materials [1]. They're fairly comprehensive and regularly updated.
[1] https://bootlin.com/docs/
Bootlin has some very nice introductory training materials online:
https://bootlin.com/doc/training/linux-kernel/
https://bootlin.com/docs/
Allwinner SoC used in PinePhone has a longstanding community around it organized around http://linux-sunxi.org/ There are a lot of materials, including datasheets there, etc.
https://wiki.pine64.org/index.php/PinePhone wiki also has a lot of information.
#pinedev at freenode is where the kernel development discussion happens, so feel free to join there.
Hire open source consulting companies like Collabora, Igalia, Bootlin etc to work on Linux/graphics stuff. IIRC Valve is doing this work via Collabora. I note that both Igalia and Bootlin have done crowdfunding campaigns to support their Linux, graphics and web work:
http://open-prioritization.igalia.com/
https://www.igalia.com/2020/09/28/Open-Prioritization-Experiment-60-day-Review.html
https://bootlin.com/blog/tag/crowdfunding/
https://github.com/fossjobs/fossjobs/wiki/Resources#employers
Bootlin's embedded Linux class is fantastic and will teach you how to do all of this--uboot setup, building a kernel and root fs, etc. All of the materials for the class are free downloads so if you're motivated you can read the slides and follow it all yourself. I highly recommend it: https://bootlin.com/training/embedded-linux/