Because I unfortunately had to deal with VBA in 2025, I share my interpretation of the solution of "the hard way" as described by @Henzi. It is important that you make sure that the Collection only contains Strings.
Function StrCollJoin( _
StrColl As Collection _
, Optional ByVal JoinStr As String = "" _
) As String
' ----------------------------------------------------------------------------
' - Performs a string join as if the collection was an array of strings
' ----------------------------------------------------------------------------
JoinedStr = ""
First = True
For Each Item In StrColl
If Not First Then JoinedStr = JoinedStr & JoinStr Else First = False
JoinedStr = JoinedStr & Item
Next Item
StrCollJoin = JoinedStr
End Function