Kody Simpson's Spigot playlist (Latest Versions of Minecraft) Extremely extensive with ≈100 videos, well-paced video tutorials that build up on difficulty each video
CodedRed's 1.15 Spigot Playlist (1.15+) Well-sized videos that go over most scenarios you will end up in and may be the only ones on youtube
NMS
Dealing with server-bound packets/net.minecraft.server
Resources that will aid you in the development of mods for Minecraft
Forge
Resources that will help you make forge mods
Forge's official Wiki An official extensive wiki for getting started with forge, allows you to pick what version of the game you want a tutorial for
Forge Tutorial 1.19 Playlist Tutorials by 'Modding by Kaupenjoe' that go through modding for 1.19 Forge comprised of 40 videos that touches on nearly every aspect of modding
Fabric
Resources that will help you make fabric mods
Fabric's official Wiki An official extensive wiki for getting started with fabric, goes through specific examples
Fabric Tutorial 1.19 Playlist Tutorials by 'Modding by Kaupenjoe' that go through modding for 1.19 Fabric comprised of 41 videos that touches on nearly every aspect of modding
Mixins
Resources that will help you with mixins
Fabric's mixin Wiki An official wiki page by Fabric which has a coherent explanation of how to use mixins, it also has examples and code snippets with explanations
Mixin's official Wiki Mixin's official wiki that delves into a deeper example and explanation of how to use mixins and how they work with diagrams
Resource Pack Development
Resources that will help you in the process of making resource packs and will give you a better understanding of how it works
General
Resources that will help you make a resource pack using every feature
Love & Tolerance Resource Pack Guide Arguably the holy bible of resources when it comes to resource packs, goes through nearly everything you need to know for making a texture pack and more such as even more resources and software
Minecraft Style Guide A guide that goes through thoroughly of the dos and don'ts of making a resource pack in the Minecraft style
Custom UI Bar Tutorial A tutorial that shows you how to make the increasingly popular custom ui bars
OptiFine Docs Documentation that will explain all OptiFine-specific texture-related things such as CIT, CEM, CTM and more.
Model Development
Resources that will help you with importing custom models into the game
Blockbench - The most popular software for making voxel-based blocks/entities for Minecraft.
Blockbench Quickstart - Tells you your first steps for learning how to use blockbench for your use
Minecraft Style Guide A guide that goes through thoroughly of the dos and don'ts of making a model in the Minecraft style
OBJMc Allows you to import animated/non-animated OBJs into Minecraft using texture packs
OBJMc Tutorial OBJMc is rather confusing and this tutorial may help with any issues you may face
Vanilla Shaders
Resources that will help aid you create vanilla shaders that were added as of 1.17
Minecraft Shaders Wiki A community ran, up-to-date resource filled with various examples, documentation, images and more! One of the largest resources.
Minecraft Shaders Repo Community documentation of shaders with image examples and explanations
Text Shader Wiki A wiki specifically on text-based vanilla shaders
Map Development
Resources that will help you in the process of making maps/worlds for Minecraft
Mods
WorldEdit(Forge/Fabric) A mod that makes building easier using: selections, schematics, brushes, shapes functions and more!
WorldEdit CUI Fabric A mod that helps you visualize selections and data of world edit
WorldEdit CUI Forge A mod that helps you visualize selections and data of world edit
Axiom(Fabric) A mod with various vanilla tweaks for building regularly as well as a visual editor with brushes, selections, operators and more!
Software
WorldPainter Software that makes sculpting and painting terrain and structures easier
Amulet Software that allows you to alter blocks, chunks and biomes and also copy and paste structures with the ability of operations and free transform