Here is a list of all documented files with brief descriptions:

docs
doxygen
codegen_macros.h	Macros that the generated precompiled header defines, listed for the reference
generated_symbols.h	Symbols that generated code and module libraries export
namespaces.h	Namespace pages for the reference
rex
app
app.h
title_app.h
apu
audio_driver.h
audio_system.h
filesystem
directory.h
file.h
file_open.h
file_stat.h
file_system.h
file_system_error.h
file_system_factory.h
mount.h
stfs.h	STFS and SVOD package layouts and container reader
xdbf.h
xex.h	XEX image reader and PE image parsing
gpu
graphics_driver.h
graphics_system.h
xenos_graphics_driver.h
hid
binding.h	Input bindings and key capture
input_driver.h	Input driver interface
input_system.h
pad.h	Pad state, capabilities and identity
kernel
object
event_object.h
file_object.h
io_completion_object.h
kernel_object.h
kernel_thread.h
module_object.h
mutant_object.h
notification_broadcaster.h
notify_listener_object.h
object.h
object_block.h
object_pin.h
object_table.h
semaphore_object.h
symbolic_link_object.h
timer_object.h
xam_object.h
types
content.h	Content package and language IDs
io.h
list.h	Guest list and spin lock structures
memory.h	Virtual memory flags and typed guest pointers
status.h
thread.h
xex.h
export.h
function_dispatcher.h
guest_allocator.h
guest_memory.h
heap.h
kernel_settings.h
memory_view.h
module.h
runtime_settings.h
shim.h
symbol_map.h
thread_registry.h
thread_state.h
title_debug_output.h
title_exit.h
title_paths.h
title_runtime.h
xboxkrnl.h
logging
log_category.h
logger.h
memory
endian.h
host.h
mapped_ptr.h
stream.h	Bit, byte and ring buffer streams over memory
virtual_memory.h
platform
dynlib.h
exception_handler.h
file.h
lock_order.h
native_window.h
perf.h
process.h	Process control, console, environment and crash reports
socket.h
sync.h	Wait objects and wait functions
thread.h
timer.h
ppc
call.h
context.h
fpu.h	Vector estimates, dot product and fused multiply add
hook.h	Hook macros and typed hooks that wrap a recompiled function
image.h	PPC image layout, embedded metadata assets and pdata decoding
kernel_hook.h
midasm.h
mmio.h	Handlers for GPU and APU register block access
recompiled_code.h
seh.h
stack.h	Scoped guest stack scratch for host code that calls into guest code
vec128.h
vmx.h
string
numeric.h
path.h
text.h	ASCII and UTF-8 text operations, the string builder and the string keys
ui
dialog.h
imgui_config.h
style.h
ui_driver.h
ui_event.h
ui_system.h	UI system, overlays and dialogs
window.h
xam
achievement_manager.h
app_manager.h	XAM app base class and the registry that routes messages by app ID
content_manager.h
device.h	Storage device types and the placeholder devices reported to the title
input.h	XInput structures and constants and the XAM error codes
language.h	Mapping of host locales to title languages
title_lifecycle.h	Kernel interfaces and lifecycle calls XAM receives for a title
user_profile.h	Profile settings and the signed in user profile
xenos
fetch.h	Fetch constant layouts, texture sampling enums and limits
format.h	Data, texture, vertex and render target formats and conversions
packet.h	Command packet opcodes, events and builders
register.h	Register layouts and the register file
state.h	Xenos pipeline state, shader and EDRAM enums
texture.h	Xenos texture info, memory layout and format conversion
ucode.h
app_info.h
assert.h
bit.h
chrono.h	Host and guest clocks over NT time
cli.h
console_command.h	Console commands an owner registers
driver.h
driver_context.h
driver_module.h
driver_registration.h
guest.h	Guest memory, write watch and interrupt interfaces
hash.h
init.h
input_keys.h
math.h
memory.h
module.h	Interfaces, factory and module declaration
overlay_renderer.h
path_settings.h
platform.h	Platform flags, packing macros and export markers
result.h
setting.h	Typed settings and the owners that declare them
settings_registry.h	The registry of every registered setting
title.h
types.h	Integer types, byte order wrappers and size literals
ui_thread.h
utility.h	Move only function, const propagating pointer and scope guard