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

include
flecs
addons
cpp
impl
field.hpp	Field implementation
iter.hpp	Iterator implementation
world.hpp	World implementation
mixins
alerts
builder.hpp	Alert builder
builder_i.hpp	Alert builder interface
decl.hpp	Alert declarations
entity_view.inl	Alerts entity mixin
impl.hpp	Alerts module implementation
mixin.inl	Alert world mixin
app
builder.hpp	App builder
decl.hpp	App addon declarations
mixin.inl	App world addon mixin
component
impl.hpp	Component mixin implementation
mixin.inl	Component mixin
doc
decl.hpp	Doc mixin declarations
entity_builder.inl	Doc entity builder mixin
entity_view.inl	Doc entity view mixin
impl.hpp	Doc mixin implementation
entity
builder.hpp	Entity builder
impl.hpp	Entity implementation
mixin.inl	Entity world mixin
entity_ranges
impl.hpp	Entity ranges implementation
mixin.inl	Entity ranges world mixin
enum
entity_view.inl	Enum entity view mixin
impl.hpp	Enum implementation
mixin.inl	Enum world mixin
event
builder.hpp	Event builder
decl.hpp	Event declarations
entity_builder.inl	Event entity mixin
entity_view.inl	Event entity mixin
impl.hpp	Event implementation
mixin.inl	Event world mixin
frame
impl.hpp	Frame implementation
mixin.inl	Frame world mixin
id
decl.hpp	ID class
impl.hpp	ID class implementation
mixin.inl	ID world mixin
json
decl.hpp	JSON addon declarations
entity.inl	JSON entity mixin
entity_builder.inl	JSON entity mixin
entity_view.inl	JSON entity mixin
iterable.inl	JSON iterable mixin
query.inl	JSON query mixin
world.inl	JSON world mixin
meta
component.inl	Meta component mixin
cursor.hpp	Cursor for reading/writing dynamic values
decl.hpp	Meta declarations
entity_builder.inl	Meta entity builder mixin
impl.hpp	Meta implementation
opaque.hpp	Helpers for opaque type registration
untyped_component.inl	Meta component mixin
world.inl	Meta world mixin
metrics
builder.hpp	Metric builder
decl.hpp	Metrics declarations
impl.hpp	Metrics module implementation
mixin.inl	Metrics world mixin
untyped_component.inl	Metrics component mixin
module
impl.hpp	Module implementation
mixin.inl	Module world mixin
observer
builder.hpp	Observer builder
builder_i.hpp	Observer builder interface
decl.hpp	Observer declarations
impl.hpp	Observer implementation
mixin.inl	Observer world mixin
pipeline
builder.hpp	Pipeline builder
builder_i.hpp	Pipeline builder interface
decl.hpp	Pipeline module declarations
impl.hpp	Pipeline module implementation
mixin.inl	Pipeline world mixin
prefab
impl.hpp	Prefab implementation
mixin.inl
query
builder.hpp	Query builder
builder_i.hpp	Query builder interface
decl.hpp	Query declarations
impl.hpp	Query implementation
mixin.inl	Query world mixin
sparse_query.hpp	Direct iteration of sparse component storages
rest
decl.hpp	Rest module declarations
impl.hpp	Rest module implementation
script
builder.hpp	Script builder
decl.hpp	Script declarations
impl.hpp	Script implementation
mixin.inl	Script world mixin
stats
decl.hpp	Stats module declarations
impl.hpp	Stats module implementation
system
builder.hpp	System builder
builder_i.hpp	System builder interface
decl.hpp	System module declarations
impl.hpp	System module implementation
mixin.inl	System module world mixin
term
builder_i.hpp	Term builder interface
decl.hpp	Term declarations
impl.hpp	Term implementation
mixin.inl	Term world mixin
timer
decl.hpp	Timer module declarations
impl.hpp	Timer module implementation
mixin.inl	Timer module mixin
system_mixin.inl	Timer module system mixin
units
decl.hpp	Units module declarations
impl.hpp	Units module implementation
utils
array.hpp	Array class
builder.hpp	Builder base class
component_id.hpp
enum.hpp	Compile-time enum reflection utilities
function_traits.hpp	Compile-time utilities to inspect properties of functions
iterable.hpp	Base class for iterable objects, like queries
map.hpp	Minimal C++ wrapper for ecs_map_t iteration
signature.hpp	Compile-time utilities for deriving query attributes from a parameter pack
string.hpp	String utility that doesn't implicitly allocate memory
stringstream.hpp	Wrapper around ecs_strbuf_t that provides a simple stringstream-like API
utils.hpp	Flecs STL (FTL?)
c_types.hpp	Aliases for types/constants from C API
component.hpp	Registering/obtaining info from components
delegate.hpp	Wrappers around C++ functions that provide callbacks for C APIs
entity.hpp	Entity class
entity_component_tuple.hpp	Utilities to fetch components as tuples from entities
entity_view.hpp	Entity class with only read-only operations
field.hpp	Wrapper classes for fields returned by flecs::iter
flecs.hpp	Flecs C++17 API
iter.hpp	Wrapper classes for ecs_iter_t and component arrays
lifecycle_traits.hpp	Utilities for discovering and registering component lifecycle hooks
log.hpp	Logging functions
pair.hpp	Utilities for working with compile-time pairs
ref.hpp	Class that caches data to speed up get operations
table.hpp	Direct access to table data
type.hpp	Utility functions for ID vector
world.hpp	World class
alerts.h	Alerts module
app.h	App addon
constraint_traits.h	Constraint traits addon
deprecated.h	The deprecated addon contains deprecated operations
doc.h	Doc module
entity_ranges.h	Entity id range management
flecs_c.h	Extends the core API with convenience macros for C applications
flecs_cpp.h	C++ utility functions
frame.h	Frame management
http.h	HTTP addon
json.h	JSON parser addon
log.h	Logging addon
meta.h	Meta addon
meta_c.h	Utility macros for populating reflection data in C
metrics.h	Metrics module
module.h	Module addon
os_api_impl.h	Default OS API implementation
pipeline.h	Pipeline module
rest.h	REST API addon
script.h	Flecs script module
script_async.h	Async/await support for Flecs script
script_event.h	Mouse and keyboard events for Flecs script
script_math.h	Math functions for Flecs script
script_platform.h	Platform constants for Flecs script
stats.h	Statistics addon
system.h	System module
timer.h	Timer module
units.h	Units module
os_api.h	Operating system abstraction API
flecs.h	Flecs public API