How to use Getopenfilename to switch between workbooks

Unfortunately doesn't seem to work
The moment I Dim them to Workbook, then the Getopenfilename line flags as an error.
I think these lines need the Dim to be string

    MEWBC = Application.GetOpenFilename(FileFilter:="Excel Files (*.xls*), *.xls*", Title:="Where is CURRENT Mth Wkbk")
    MEWBP = Application.GetOpenFilename(FileFilter:="Excel Files (*.xls*), *.xls*", Title:="Where is PRIOR Mth Wkbk")

Ok, so Im trying to do something like this..

Dim the workbook as a different name
Open the workbook, then assign the workbook variable

Still can't get it to work though I'm thinking that perhaps "ThisWorkbook" is not the correct command to use repeatedly?

    Dim BillCalcs As Workbook
    Set BillCalcs = ThisWorkbook
    
    Dim MEWBC As String
    Dim MEWBP As String
    
    MEWBC = Application.GetOpenFilename(FileFilter:="Excel Files (*.xls*), *.xls*", Title:="Where is CURRENT Mth Wkbk")
    MEWBP = Application.GetOpenFilename(FileFilter:="Excel Files (*.xls*), *.xls*", Title:="Where is PRIOR Mth Wkbk")
    
    Dim wbC As Workbook
    Dim wbP As Workbook
    
    Workbooks.Open (MEWBC), UpdateLinks:=0
    Set wbC = ThisWorkbook
    
    Workbooks.Open (MEWBP), UpdateLinks:=0
    Set wbP = ThisWorkbook
    
    Workbooks(wbC).Activate  'code executes up to this point, and then stops with the debugger highlighting this line
    Sheets("A.4 Engagement Summary").Select
    range("B13").Select
    range(Selection, Selection.End(xlDown)).Select
    Selection.Copy
    
    Workbooks(BillCalcs).Activate
    ActiveSheet.Select
    Selection.Paste

I've tried substituting the "workbooks" command with "windows" but that also doesn't work. Eg. "Windows(BillCalcs).Activate"

Ok, managed to get it to work through a bit more research and trial and error.
Here is the solution for anybody else's reference:

Sub codetest()
   
    Dim BillCalcs As Workbook
    Set BillCalcs = ActiveWorkbook
    
    Dim MEWBC As String
    Dim MEWBP As String
    
    MEWBC = Application.GetOpenFilename(FileFilter:="Excel Files (*.xls*), *.xls*", Title:="Where is CURRENT Mth Wkbk")
    MEWBP = Application.GetOpenFilename(FileFilter:="Excel Files (*.xls*), *.xls*", Title:="Where is PRIOR Mth Wkbk")
    
    Dim wbC As Workbook
    Dim wbP As Workbook
    
    Set wbC = Workbooks.Open(MEWBC)
    Set wbP = Workbooks.Open(MEWBP)
    
    wbC.Activate
    Sheets("CUSTOMSHEETNAME").Select
    range("B13").Select
    range(Selection, Selection.End(xlDown)).Select
    Selection.Copy
    
    BillCalcs.Activate
    ActiveSheet.Select
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False
    
End Sub

Still struggling with this And I'm so sure it is a small fix that's required.
Anybody?