QGIS 3: Layout expression to get all features of a layer within the current map view

In a map layout template you can insert an expression which gets all feature's names of a certain layer, which are contained in the current layout's view/extent. For wxample you could concatenate all feature's names contained in the current map view like so:

aggregate(layer:='Corridors_86ef2ea9_d1a1_4d9f_9735_7b7b2fb54cb2', 
aggregate:='concatenate', 
expression:="Name", filter:=within($geometry, map_get( item_variables('Main Map'), 
'map_extent')), concatenator:=', ')

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))) 

Return Excel Row Indices of non-empty Cells in Column

Column with Data in A1:A20

Formula in Cell B1

=WENNFEHLER(AGGREGAT(15;6;ZEILE($A$1:$A$20)*N(LÄNGE($A$1:$A$20)>0);SUMMENPRODUKT(N(LÄNGE(Datenablage!$A$1:$A$20)=0))+ZEILE());"")

SUMMENPRODUKT(N(LÄNGE(Datenablage!$A$1:$A$20)=0)) is needed because each LEN=0 will be a smallest value, so if you have 3 cells with LEN=0 the ksmallest with k=4 will be the first non empty value..

..for details on aggregate check: https://www.youtube.com/watch?v=He3dblboncw