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

[detail level 1234]

► alg
armadillo_headers.h
gdal_alg.h	Public (C callable) GDAL algorithm entry points, and definitions
gdal_alg_priv.h
gdal_simplesurf.h	Class for searching corresponding points on images
gdalgrid.h	GDAL gridder related entry points and definitions
gdalgrid_priv.h
gdallinearsystem.h
gdalmatching.cpp	Algorithms for searching corresponding points on images
gdalpansharpen.h	GDAL pansharpening related entry points and definitions
gdalwarper.h	GDAL warper related entry points and definitions
gdalwarpkernel_opencl.h
gvgcpfit.h
internal_qhull_headers.h
thinplatespline.h
► apps
commonutils.h
gdal_utils.h	Public (C callable) GDAL Utilities entry points
gdal_utils_priv.h
► frmts
► vrt
gdal_vrt.h	Public (C callable) entry points for virtual GDAL dataset objects
vrtdataset.h
► gcore
gdal.h	Public (C callable) GDAL entry points
gdal_frmts.h
gdal_mdreader.h
gdal_pam.h
gdal_priv.h	C++ GDAL entry points
gdal_proxy.h
gdal_rat.h
gdal_thread_pool.h
gdal_version.h
gdalexif.h
gdalgeorefpamdataset.h
gdaljp2abstractdataset.h
gdaljp2metadata.h
gdaljp2metadatagenerator.h
gdalpython.h
gdalsse_priv.h
nasakeywordhandler.h
► gnm
gnm.h
gnm_api.h
gnm_priv.h
gnmgraph.h
► ogr
► ogrsf_frmts
► generic
ogr_gensql.h
ogreditablelayer.h
ogremulatedtransaction.h
ogrlayerdecorator.h
ogrlayerpool.h
ogrmutexeddatasource.h
ogrmutexedlayer.h
ogrunionlayer.h
ogrwarpedlayer.h
ogr_attrind.h
ogrsf_frmts.h	Classes related to registration of format support, and opening datasets
ogr_api.h	C API and defines for OGRFeature, OGRGeometry, and OGRDataSource related classes
ogr_core.h	Core portability services for cross-platform OGR code
ogr_expat.h
ogr_feature.h	Simple feature classes
ogr_featurestyle.h	Simple feature style classes
ogr_geo_utils.h
ogr_geocoding.h	C API for geocoding client
ogr_geometry.h	Simple feature geometry classes
ogr_geos.h
ogr_libs.h
ogr_p.h
ogr_proj_p.h
ogr_sfcgal.h
ogr_spatialref.h	Coordinate systems services
ogr_srs_api.h	C spatial reference system services and defines
ogr_srs_esri_names.h
ogr_swq.h
ogr_xerces.h
ogr_xerces_headers.h
ograpispy.h	OGR C API spy
ogrgeomediageometry.h
ogrpgeogeometry.h
► port
cpl_alibaba_oss.h
cpl_atomic_ops.h
cpl_auto_close.h
cpl_aws.h
cpl_azure.h
cpl_compressor.h	API for compressors and decompressors of binary buffers
cpl_config.h
cpl_config_extras.h
cpl_conv.h	Various convenience functions for CPL
cpl_cpu_features.h
cpl_csv.h
cpl_curl_priv.h
cpl_error.h	CPL error handling services
cpl_error_internal.h
cpl_google_cloud.h
cpl_hash_set.h	Hash set implementation
cpl_http.h	Interface for downloading HTTP, FTP documents
cpl_json.h	Interface for read and write JSON documents
cpl_json_header.h
cpl_json_streaming_parser.h
cpl_json_streaming_writer.h
cpl_list.h	Simplest list implementation
cpl_md5.h
cpl_mem_cache.h
cpl_minixml.h	Definitions for CPL mini XML Parser/Serializer
cpl_minizip_ioapi.h
cpl_minizip_unzip.h
cpl_minizip_zip.h
cpl_multiproc.h
cpl_odbc.h	ODBC Abstraction Layer (C++)
cpl_port.h	Core portability definitions for CPL
cpl_progress.h
cpl_quad_tree.h	Quad tree implementation
cpl_sha1.h
cpl_sha256.h
cpl_spawn.h
cpl_string.h	Various convenience functions for working with strings and string lists
cpl_swift.h
cpl_time.h
cpl_userfaultfd.h
cpl_vax.h
cpl_virtualmem.h	Virtual memory management
cpl_vsi.h	Standard C Covers
cpl_vsi_error.h
cpl_vsi_virtual.h
cpl_vsil_curl_class.h
cpl_vsil_curl_priv.h
cpl_vsil_plugin.h
cpl_worker_thread_pool.h	Class to manage a pool of worker threads
cplkeywordparser.h
gdal_csv.h