Extracting Resources

In order to create mods, you will need to have the game's resources extracted into the various files that can be edited. Resources are divided by levels or general categories. They will be sorted into folders when extracted.

Extraction Tools:

EternalResourceExtractor by PowerBall253 - Download
Command-line tool. Exports everything, but textures/models are exported as "headers" only - not for texture mods.
CMD Syntax: EternalResourceExtractor.exe [path to .resources file] [out path] [options]
[Documentation & Source Code]

SAMUEL Asset Extraction Tool by SamPT - Download 
GUI tool. Exports textures, .decls, and .entities files.
[Documentation & Source Code]

VEGA by DTZxPorter - Download
GUI tool. Exports textures and models. Preferred tool for texture modding.

Make sure you have enough disk space when extracting resources. As of Game Update 6.66 rev 2, EternalResourceExtractor requires 20.0 GB of free space to extract everything. SAMUEL or VEGA require even more space, since they export full-size images. If using those tools, you should only export the textures you need.

List of Resources:

gameresources contains general information that will be used for all levels.
You will be using this resource frequently.

Resource	Description
gameresources	General
e1m1_intro	Hell on Earth
e1m2_battle	Exultia
e1m3_cult	Cultist Base
e1m4_boss	Doom Hunter Base
e2m1_nest	Super Gore Nest
e2m2_base	ARC Complex
e2m3_core	Mars Core
e2m4_boss	Sentinel Prime
e3m1_slayer	Taras Nabad
e3m2_hell	Nekravol
e3m2_hell_b	Nekravol - Part II
e3m3_maykr	Urdak
e3m4_boss	Final Sin
e4m1_rig	UAC Atlantica Facility
e4m2_swamp	The Blood Swamps
e4m3_mcity	The Holt
e5m1_spear	The World Spear
e5m2_earth	Reclaimed Earth
e5m3_hell	Immora
e5m4_boss	The Dark Lord
e6m1_cult_horde	Horde Mode - Cultist Base
e6m2_earth_horde	Horde Mode - Reclaimed Earth
e6m3_mcity_horde	Horde Mode - The Holt
hub	Campaign Hub
dlc_hub	ARC Carrier
shell	Main Menu
pvp_bronco	Battlemode - Celestial
pvp_darkmetal	Battlemode - Corrosion
pvp_deathvalley	Battlemode - Penance
pvp_inferno	Battlemode - Tundra
pvp_laser	Battlemode - Forsaken
pvp_shrapnel	Battlemode - Torment
pvp_sideswipe	Battlemode - Stronghold
pvp_thunder	Battlemode - Armageddon
pvp_zap	Battlemode - Extraction
tutorial_sp	Slayer Tutorial
tutorial_demons	Demon Player Tutorial
tutorial_pvp_laser	Battlemode - Tutorial
warehouse	Skins
meta	Contains "container.mask"

There are folders with "_patch_#" appended at the end and they contain overriding files for their parent folder.
The patch number does not always mean it has a high priority because DOOM Eternal often changes the priorities with each update.