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

[detail level 1234]

lix
libcmd
built-path.hh
cmd-profiles.hh
command.hh
common-eval-args.hh
editor-for.hh
installable-attr-path.hh
installable-derived-path.hh
installable-flake.hh
installable-value.hh
installables.hh
legacy.hh
markdown.hh
repl-interacter.hh
repl.hh
libexpr
flake
flake.hh
flakeref.hh
lockfile.hh
value
context.hh
attr-path.hh
attr-set.hh
eval-cache.hh
eval-error.hh
eval-inline.hh
eval-settings.hh
eval.hh
extra-primops.hh
function-trace.hh
gc-alloc.hh
gc-small-vector.hh
get-drvs.hh
json-to-value.hh
nixexpr.hh
pos-idx.hh
pos-table.hh
primops.hh
print-ambiguous.hh
print-options.hh	Options for printing Nix values
print.hh	Common printing functions for the Nix language
repl-exit-status.hh
search-path.hh
symbol-table.hh
value-to-json.hh
value-to-xml.hh
value.hh
libfetchers
attrs.hh
builtin-fetchers.hh
cache.hh
fetch-settings.hh
fetch-to-store.hh
fetchers.hh
registry.hh
libmain
common-args.hh
crash-handler.hh
loggers.hh
progress-bar.hh
shared.hh
libstore
build
child.hh
derivation-goal.hh
drv-output-substitution-goal.hh
goal.hh
hook-instance.hh
local-derivation-goal.hh
personality.hh
substitution-goal.hh
worker.hh
builtins
buildenv.hh
binary-cache-store.hh
build-result.hh
builtins.hh
common-protocol-impl.hh
common-protocol.hh
content-address.hh
crypto.hh
daemon.hh
derivations.hh
derived-path-map.hh
derived-path.hh
downstream-placeholder.hh
dummy-store.hh
filetransfer.hh
fs-accessor.hh
gc-store.hh
globals.hh
http-binary-cache-store.hh
indirect-root-store.hh
legacy-ssh-store.hh
length-prefixed-protocol-helper.hh
local-binary-cache-store.hh
local-fs-store.hh
local-store.hh
lock.hh
log-store.hh
machines.hh
make-content-addressed.hh
names.hh
nar-accessor.hh
nar-info-disk-cache.hh
nar-info.hh
outputs-spec.hh
parsed-derivations.hh
path-info.hh
path-references.hh
path-regex.hh
path-with-outputs.hh
path.hh
pathlocks.hh
profiles.hh
realisation.hh
remote-fs-accessor.hh
remote-store-connection.hh
remote-store.hh
s3-binary-cache-store.hh
s3.hh
serve-protocol-impl.hh
serve-protocol.hh
sqlite.hh
ssh-store.hh
ssh.hh
store-api.hh
store-cast.hh
temporary-dir.hh
uds-remote-store.hh
worker-protocol-impl.hh
worker-protocol.hh
libutil
abstract-setting-to-json.hh
ansicolor.hh	Some ANSI escape sequences
apply-config-options.hh	Options for applying Config settings
archive.hh
args.hh
async-collect.hh
async-io.hh
async-semaphore.hh	A semaphore implementation usable from within a KJ event loop
async.hh
backed-string-view.hh
box_ptr.hh
canon-path.hh
cgroup.hh
charptr-cast.hh
checked-arithmetic.hh
chunked-vector.hh
closure.hh
comparator.hh
compression.hh
compute-levels.hh
concepts.hh
config-impl.hh
config.hh
current-process.hh
deprecated-features.hh
english.hh
environment-variables.hh
error.hh	This file defines two main structs/classes used in nix error handling
escape-char.hh
escape-string.hh
exit.hh
experimental-features.hh
file-descriptor.hh
file-system.hh
finally.hh
fmt.hh
generator.hh
git.hh
hash.hh
hilite.hh
input-accessor.hh
json-fwd.hh
json.hh
logging.hh
lru-cache.hh
manually-drop.hh
monitor-fd.hh
mount.hh
namespaces.hh
notifying-counter.hh
pool.hh
position.hh	Pos and AbstractPos
print-elided.hh
processes.hh
ref.hh
references.hh
regex-combinators.hh
regex.hh
repair-flag.hh
result.hh
serialise.hh
shlex.hh
signals.hh
source-path.hh	SourcePath
split.hh
strings.hh
suggestions.hh
sync.hh
tarfile.hh
terminal.hh
thread-name.hh
thread-pool.hh
topo-sort.hh
tracepoint.hh
types.hh
unix-domain-socket.hh
url-name.hh	, for some hueristic-ish URL parsing
url-parts.hh
url.hh
users.hh
variant-wrapper.hh
xml-writer.hh
nix
add-to-store.hh
build.hh
bundle.hh
cat.hh
config.hh
copy.hh
daemon.cc
daemon.hh
derivation-add.hh
derivation-show.hh
derivation.hh
develop.hh
diff-closures.hh
doctor.hh
dump-path.hh
edit.hh
eval.hh
flake.hh
fmt.hh
hash.hh
log.hh
ls.hh
make-content-addressed.hh
nar.hh
optimise-store.hh
path-from-hash-part.hh
path-info.hh
ping-store.hh
prefetch.hh
profile.hh
realisation.hh
registry.hh
repl.hh
run.hh
search.hh
sigs.hh
store-copy-log.hh
store-delete.hh
store-gc.hh
store-repair.hh
store.hh
upgrade-nix.hh
verify.hh
why-depends.hh
tests
unit
libstore
protocol.hh