How to quickly switch what worksheet/workbook is being edited in VBA - vba

Question

Basically, I want my program to open a hyperlink to another workbook and copy a range of values onto my original workbook. The problem is, I don't know how to properly reference the workbook to "paste" onto
I've tried to replicate a lot of different internet examples. I've tried using "Set" and "With" to switch between them and I've tried to make the original workbook the active workbook before pasting and nothing has been working.
Private Sub findColumns_button_Click() 'userform code
Dim index As Integer, hyperlink_A As Variant, Wb As Worksheet, mainWorkbook As Variant
mainWorkbook = ActiveWorkbook.FullName
Worksheets("Data").Cells(2, 2) = ResultA_Combo.Value
index = findIndex(Cells(2, 2).Value)
hyperlink_A = findHyperlink(index)
Worksheets("Data").Cells(3, 2) = hyperlink_A
Workbooks.Open Filename:=hyperlink_A 'opens correctly
Dim test As Double
test = Worksheets("Data").Range("S23")
MsgBox test 'displays correct value from desired workbook
Workbooks.Activate Filename:=mainWorkbook 'not sure what to do here
End Sub
MsgBox test returns the desired value but I can't find a way to set a cell value in the original workbook equal to test

Answer 1

See if this helps, I've edited your code, and by comparing I hope you get in the right direction... see comments in code for more details:
Private Sub findColumns_button_Click() 'userform code
Dim index As Integer, hyperlink_A As Variant, Wb As Worksheet, mainWorkbook As Workbook
Dim wbSrc As Workbook
Set mainWorkbook = ActiveWorkbook
With mainWorkbook.Worksheets("Data")
.Cells(2, 2) = ResultA_Combo.Value
index = findIndex(.Value)
hyperlink_A = findHyperlink(index)
.Cells(3, 2) = hyperlink_A
End With
Set wbSrc = Workbooks.Open(Filename:=hyperlink_A) 'Allocate to variable and open at the same time
Dim test As Double
test = mainWorkbook.Worksheets("Data").Range("S23")
MsgBox test 'displays correct value from first workbook
mainWorkbook.Activate 'as long you fully qualify the ranges, you don't need and should avoid as much as possible to use .Activate & .Select
Dim test2 As Double
wbSrc.Worksheets(1).Range("A1") = mainWorkbook.Worksheets("Data").Range("S23") * 2
test2 = wbSrc.Worksheets(1).Range("A1")
MsgBox test2 'displays correct value from second workbook
End Sub