For most of these types of tasks, there isn’t tutorials (yet), although for modified structures there is a written tutorial on the forums here, and there is also as I recall a number of guides relating to things you mentioned. Just apply the things taught in Azreal’s video to the rest of the items you want to mod. Editing beacon drop lists is just a matter of modifying their drop contents and remapping them I believe, but I don’t know for sure, as I’ve never modded anything with drop editing.
As for creating new items, generally, it’s a smart play to use an existing item in the game already, duplicate all its assets, and then rename & remap them to the new duplicated version. Unless you know what you are doing, I would NOT recommend trying to make an item completely from scratch, and even then 95% of the time you should be able to find an item that is suitable for streamlining in your new modded item.
Most of the ADK doesn’t come with a user manual unfortunately, so most of this stems from experience.
You won’t find specific tutorials on how we created the mods we are working on. At least not until the competition is over. Then we’ll probably go nuts on putting out how we did it.