You can tell that #1 goes in the BaseInventory blueprint because you’re calling an OnOverlap event with the Sphere1 component (you can see it in parenthesis in the picture). Before that image, Hakabane instructed you to make a blueprint called BaseInventory and add a Sphere in it.
As for the rest - take a look at the first image again. See the three nodes that have grey borders and a blue gear in the top left corner? Those are macros. It technically doesn’t matter where you put these - Hakabane puts them in his BaseInventory blueprint, but I put mine in my character blueprint because I prefer it that way (for now at least). The reason it doesn’t matter is because you’re casting from the BaseItem blueprint - that is to say, you’re pulling info from the BaseItem blueprint in order to use these macros. For the sake of clarity - all 3 macros are found inside the BaseInventory blueprint in Hakabane’s tutorial.
The function at the end is created just like the macros (it’s also the same way you make new variables). The function also goes into the BaseInventory blueprint.
You’re making this into Class blueprints so they aren’t dependent on the level. It would be a huge pain in the **** to re-build these scripts for every level in your game, hence nothing Hakabane has introduced should go into the level blueprint.