Which of Time Capsule do you have (the "flat" version or the "tall tower")?
onto the existing system. This aims to keep the hardware compatible with future macOS versions that may drop older protocols. Limitations
: This is the most granular technical breakdown available. It details the memory offsets for the compressed NetBSD kernels, identifies the hardware as Marvell Kirkwood based (A1409 models), and explains the signature verification processes Apple uses to prevent third-party firmware.
Most current "custom firmware" for Time Capsules is not a simple file you upload via the AirPort Utility ; it typically requires soldering or deep command-line knowledge.