In any case, this guide will eventually cover only the most basic elements of creating an item and creating an entity (a moving thing like a villager, sheep, zombie, etc.), and distributing the resulting mod package. Or perhaps you want to 'fork' a favorite mod to update for newer versions of Minecraft. Or look into how to make a resource pack if you simply want to customize textures (colors and designs), models, music, sounds, languages, splashes, fonts, or the end poem. If you simply want to add custom advancements, functions, loot tables, structures, recipes or tags to your game, look into how to make a Data pack, which does not require programming. Presumably, you already have an idea for a mod you want to create.
Mods (short for 'modifications') can modify or add items, blocks, entities, and much more.
