You need to place the blueprint with the array into the scene and then you can add static mesh actors to the array by picking from a drop down list or by using the eyedropper tool
You can’t pick them from within the blueprint editing panel. It needs world context to gather the actors.
You could can only populate the array from within the blueprint editor via begin play through a get actor of class or by tag at runtime.
I made a variable, turned on the visibility of layouts on the level and added geometry, is that all? but for some reason in the blueprint itself nothing changed in the index
The internal of the blueprint exists outside of the world context of the level.
The blueprint placed inside of the level is an instance of the blueprint that because it is inside of the level can now reference other static mesh actors that exist along side it inside of the confines of the level.
Think of it this way. Your blueprint is like a fish inside of a bag with some water. It doesn’t see the aquarium or other fishes, so it can’t interact with them.
Once you put the fish in an aquarium where there are other fishes and coral etc it can now interact with them.
This is the same process. The blueprint is isolated to itself when you edit it.
You can use print strings to check the gathered actors