How to combine a code and a macro

Hello folks,
I have a code and recorded macro. Could you please help me to merge the codes into one to be able to work with different range of data set?

Code #1: Combine files in a folder into a workbook

Sub MergeFiles()
    Dim x As Integer
        Const strDir = "C:\Mydoc\"
    Dim ThisWB As Workbook
    Dim wb As Workbook
    Dim ws As Worksheet 
    Dim strWS As String
    Dim sFileName As String
    Set ThisWB = ActiveWorkbook
    
    With Application.FileDialog(msoFileDialogOpen)
    
        .InitialFileName = strDir
        .Filters.Clear
        .Filters.Add "CSV Files", "*.csv"
        .Application.DisplayAlerts = False
        If .Show Then
            For x = 1 To .SelectedItems.Count
                Set wb = Workbooks.Open(.SelectedItems(x))
                sFileName = .SelectedItems(x)
                Set ws = ThisWB.Worksheets.Add(before:=ThisWB.Worksheets(1))
                ws.Name = ActiveSheet.Name
                wb.Sheets(1).UsedRange.Copy ws.Range("A1")
                wb.Close False
            Next x
        Else
            MsgBox "There were no files found."
        End If
    End With
End Sub

Code #2: A recorded macro to convert 5 columns worksheet to 3 columns (Combining YMD columns into "Date" column)
The problem here is, I have to run the code for each worksheet separately, and I don't know how to do it for a workbook since the data range is not fixed.
I couldn't use the "Range("A" & Rows.Count).End(xlUp).Row" function correctly.

'2nd code for each sheet! How to do it for all sheets in a workbook?
Sub Macro5to3col()
'A recorded macro 
'ColA = ID, ColB:D = Year,Month,Day & ColE = Value
'The goal is: Deleting ID, and combining YMD cols into "Date" column
    
    Columns("A:A").Select
    Selection.Delete Shift:=xlToLeft    'delete column A
    Columns("D:D").Select               'insert two columns
    Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
    Selection.Insert Shift:=xlToRight, CopyOrigin:=xlFormatFromLeftOrAbove
    Range("D1").Select
    ActiveCell.FormulaR1C1 = "Datetemp"    'a temporary Date column to avoid #REF! error
    Range("D2").Select
    ActiveCell.FormulaR1C1 = "=DATE(RC[-3],RC[-2],RC[-1])"   '=Date(Y,M,D) function
    Selection.AutoFill Destination:=Range("D2:D23012")       'The range is not fixed
    Range("D2:D23012").Select
    Selection.Copy
    Range("E2").Select
    Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False         'Copy "Datetemp" column & paste as value in next column
    Application.CutCopyMode = False
    Selection.NumberFormat = "d/m/yyyy"
    Range("E1").Select
    ActiveCell.FormulaR1C1 = "Date"
    Columns("A:D").Select                 'delete YMD & Datetemp columns
    Selection.Delete Shift:=xlToLeft
End Sub