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

Excel VBA User Defined Function for Transformation of Braun-Blanquet Values to Precentages of Vegetation Cover

Function Transf_BraunBlanquet(ByVal BB_Str As String) As String

'Transformation of Braun-Blanquet 'Artmächtigkeit' to percentage cover (similar to usage in TurboVeg or twinspan)
'The key value mapping can be altered depending on specific requirements
'This UDF is used in the UDF SumKum_BraunBlanquet(), which will apply the Transformation on a range of values and
'will sum the transformed percentages. This cumulative sum can be used to check if the Braun-Blanquet estimation for
'a vegetation layer is reasonable.

    With CreateObject("Scripting.Dictionary")
        '~~> first transfer your list in Dictionary
        .Add "r", "0"
        .Add "+", "0"
        .Add "1", "1"
        .Add "2m", "2"
        .Add "2a", "10"
        .Add "2b", "20"
        .Add "3", "37,5"
        .Add "4", "67,5"
        .Add "5", "87,5"
        
        If Len(BB_Str) = 0 Then
        '~~> case: empty cell
            Transf_BraunBlanquet = 0
            Exit Function
        End If
        
        For Each elem In .keys
            key = elem
            If key = BB_Str Then
                Transf_BraunBlanquet = .Item(elem) * 1
                Exit Function
            End If
        Next elem
        
    End With
    
End Function


Function SumKum_BraunBlanquet(Rng As Range) As Double
'See comments on Transf_BraunBlanquet() for explanations

    Dim Sum As Double
    Dim RngArr As Variant
    
    RngArr = Application.Transpose(Rng) 'dumps range values to array
    
    For Each elem In RngArr
        Sum = Sum + Transf_BraunBlanquet(elem)
    Next elem
    
    SumKum_BraunBlanquet = Sum
    
End Function