A list of resources that may help with modpack development. Also see LambdAurora's OptiFine alternatives, or Modrinth's list of modpack tools (I'll also shoutout PW-GUI, which is a tool that I use daily to develop an upcoming modpack).
Note
This list focuses on what mods can do, but you should also be familiar with the existing vanilla tools for your version. The Minecraft Wiki has tutorials on how to structure datapacks (loot tables, recipes, world generation, advancements, etc.) and resource packs (textures, language files, sounds, etc.). Also see Misode's data pack generators!
These tools allow for adding basic blocks and items without needing to write a mod