multi dimensional array with different elements; VBA excel

Use a Variant array.

Dim values() As Variant

Now, your code is making assumptions that should be removed.

BT = Range("A12") '<~ implicit: ActiveSheet.Range("A12").Value

If you mean to pull the value of A12 from a particular specific worksheet, then you should qualify that Range member call with a proper Worksheet object. See CodeName: Sheet1 for more info, but long story short if that sheet is in ThisWorkbook you can do this:

BT = Sheet1.Range("A12").Value

And now assumptions are gone. Right? Wrong. BT isn't declared (at least not here). If it's declared and it's not a Variant, then there's a potential type mismatch error with that assignment. In fact, the only data type that can accept any cell value, is Variant:

Dim BT As Variant
BT = Sheet1.Range("A12").Value

Here, we're assuming BT is a numeric value:

ReDim IT(BT) As String

That's another assumption. We don't know that BT is numeric. We don't even know that it's a value that can be coerced into a numeric data type: we should bail out if that's not the case:

If Not IsNumeric(BT) Then
    MsgBox "Cell A12 contains a non-numeric value; please fix & try again."
    Exit Sub
End If

ReDim IT(BT) As String

Now that will work... but then, only the upper bound is explicit; is this a 0-based or a 1-based array? If the module says Option Base 1, then it's 1-based. Otherwise, it's 0-based - implicit array lower bounds are an easy source of "off-by-one" bugs (like how you're populating the arrays starting at index 1, leaving index 0 empty). Always make array bounds explicit:

ReDim IT(1 To BT) As String

Unclear why you need 3 arrays at all, and why you're only populating (i,i) in the 3rd one - you cannot populate a 2D array with a Do...Loop structure; you need every value of y for each value of x, and unless you hard-code the width of the array, that's a nested loop.

Moreover, looping on the ActiveCell and Selecting an Offset is making the code 1) very hard to follow, and 2) incredibly inefficient.

Consider:

Dim lastRow As Long
lastRow = Sheet1.Range("B" & Sheet1.Rows).End(xlUp).Row

ReDim values(1 To lastRow, 1 To 2) As Variant

Dim currentRow As Long
For currentRow = 2 To lastRow
    Dim currentColumn As Long
    For currentColumn = 1 To 2
        values(currentRow, currentColumn) = Sheet1.Cells(currentRow, currentColumn).Value
    Next
Next

Now, if we don't need any kind of logic in that loop and all we want is to grab a 2D variant array that contains every cell in B2:B???, then we don't need any loops:

Dim values As Variant
values = Sheet1.Range("A2:B" & lastRow).Value

And done: values is a 1-based (because it came from a Range), 2D variant array that contains the values of every cell in A2:B{lastRow}.

Note, code that consumes this array will need to avoid assumptions about the data types in it.

As @SJR has said, variant will allow for this. The below example is a easy example how to add different types to an array. Instead of x or y you can have a cell on a worksheet.

Dim array1() As Variant, i As Long
Dim x As String, y As Long

    x = "5"
    y = 1

    For i = 1 To 10

        ReDim Preserve array1(1 To 2, 1 To i)
        array1(1, i) = x
        array1(2, i) = y
        y = y + 1
        Debug.Print array1(1, i) & "," & array1(2, i) ' This is where you insert output

    Next

You can do this:

BT = Range("A12")
ReDim IT(BT) As String
ReDim RBT(BT) As Integer

Dim IT_RBT(1 to 2) 'variant
IT_RBT(1) = IT   'add String array
IT_RBT(2) = RBT  'add Integer array

... this will keep your typed arrays functional but it's not a 2D array and you'd need to use notation like

IT_RBT(1)(1)  'String type
IT_RBT(2)(1)  'Integer type