Aggregation of Different Layers in QGIS 3.x with Field Calculator Expression Alone!!

Here's an example of how to "spatially aggregate" Polygon attribute values over features of a point layer by intersecting the two layers. Go in "Edit mode" with the target layer and check "Virtual Field" with type "decimal". Then use this in expression builder, where "layer" is the point layer, and aggregate the attribute values that intersect the target layer's geometry. With "Expression" you can define the attribute, that should aggregated!

aggregate(layer:='BIKFFH_Karwendel BIKFFH_PL', aggregate:='sum', expression:="LNUMMER", 
filter:=intersects($geometry,geometry(@parent)))