Content of partitions A_kernel/B_kernel on Orin AGX

I am trying to understand what goes into partitions A_kernel/A_kernel-dtb on the Orin AGX.
According to the boot mode, if extlinux fails, L4T will load the kernel/DT from A_kernel/A_kernel-dtb.
When looking at the log from tegraflash, the file written is boot.img. What is the format/content of this file? Is it a kernel + rootfs? If so, what goes into it exactly and what is its purpose?

kernel is the kernel image. Original file is Linux_for_Tegra/kernel/Image.

kernel-dtb is the kernel dtb. Original file depends on the sku of your board. But the file will be under Linux_for_Tegra/kernel/dtb.

Is the file going into A/B_kernel just the kernel image, or is it a kernel + initramfs? If the later, can you confirm the format of the file?
I am starting to look at Secure Boot and assume I will need to use those partitions rather than extlinux to select the kernel/dt, hence need to understand exactly what goes in them.

Answering my own question, the A/B_kernel partitions need to be an Android-style image containing the kernel + initramfs.