Own items in Minecraft?

I would like to add my own extra items with their own customized effects, durability, appearance, etc. In Minecraft, in the Java version (currently 1.16.5). Do any of you know how to do that? Maybe also with an easily understandable description?

Here you can edit items very well, but you can't change the texture. As far as I know, you have to do that with a texture pack or a mod.

http://mapmaking.fr/give1.16/

What do the input fields mean, so what do they do? I don't know my way around that well

But you definitely helped me a bit, thank you!