Read in data from one file to another

my Name is Sebastian. I'm from Germany.
I have to tell my problem here, because the german side is currently down

my Problem:
I have 2 Excel files. One with the macro-code and another one.
The one with the macro is named "Book1.xlsx" and the other one is named "Book2.xlsx".
The name of the second file (in this case Book2) change weekly. But the name of the sheet in which are my data is always the same (in this case "sheet1").

For my understanding:
- I start the macro in Book1.
- The macro ask me for the other file with the data i need
- I open the file (Book2 in this case)
- Now the macro should read out data from Book2(table) an put it into Book1 (table)
- After read out a area with no entry --> read out should finish or after 16 columns.
- After read out the files from the table the macro should close the Book2.


This is my file, can anyone take a look at this?

Sub test1()
Dim fileselection As Variant

again:
fileselection = Application.GetOpenFilename

If fileselection <> False Then
    Workbooks.Open Filename:=fileselection
     Else
    If MsgBox("You chosse no file! Press 'OK' to choose a file, or 'Abbrechen' to cancel.", (vbOKCancel)) = vbOK Then
        GoTo again
        End If
        End If

'readIn 1 column

    Dim i As Integer
    For i = 4 To 16

    Windows("Book1.xlsx").Activate
    Range("C" & i).Select
    ActiveCell = "='[fileselection]Sheet1 (sel)'!C" & i 

'readIn 2 column

    Windows("Book1.xlsx").Activate
    Range("D" & i).Select
    ActiveCell = "='[fileselection]Sheet1 (sel)'!D" & i 


 Next i
      
End Sub

one more question, can i declare two variables? like i & j ?
Like: i is from 4 to 16 and j is from 6 to 18 ? or should i write "i+2" instead of j ?

sorry for my bad english. I try my best