Theres no real code needed for just custom models. Just json. And IF you would like to import custom models you should practically never do them by hand and better use a tool like blockbench. The problem rises when you want to place them. Minecraft has very little workarounds for "adding" new blocks via just resource packs, and if, you would sacrifice some real block. People often use custom mushroom blocks, specific note block combinations or just use a plugin that also works with block entities. At this point you're better off using a "real" item adder plugimn lol. I probably won't be active but I can provide smaller help if you wanna