Earlier releases had U-Boot, but as time went by, CBoot absorbed some of U-Boot’s functionality, and U-Boot itself was removed. You’ll be interested in CBoot, not U-Boot (see the official docs for your release).
If you download the kernel source from the L4T URL, then you’ll find three subdirectories. The kernel itself is in the “kernel/” subdirectory. You’ll find a subdirectory for bootloader and other firmware. During build of the Linux kernel itself some features, which depend on that content, use a relative path which exits the Linux source and refers to that other code. Similarly, there might be some content in those other directories which accesses the Linux kernel source via relative path.
Linux kernel drivers have a lot of development documentation “out in the wild”. This isn’t specific to Jetsons, although you need arm64/aarch64 architecture. Boot content though is custom to the Jetson (when you get to newer hardware using UEFI…which TX2s cannot support…then things start getting “standardized”). Much of the boot content is “black box”, but the drivers you have access to are bare metal targets just like the Linux kernel. In fact much of the driver code in booting is nearly identical to kernel drivers.
You’ll probably want someone from NVIDIA answering specific questions related to anything in boot, and possibly in the Linux kernel, but you’ll need to describe a specific use-case.