One can address a specific cell directly by row/column index. E.g. like
firstTable.Rows.Cells.Range.Text = "My Value for that Cell";
Note: Keep in mind that for office interop the index of a collection starts at '1' and ends at 'collectionName.Count'
Just played around more with word interop and found a second (maybe slightly more performant) way how to address a cell of a table:
secondTable.Cell(2, 1).Range.Text = "My 2/1";
See here for more details: Table.Cell(Int32, Int32) Method (Microsoft.Office.Interop.Word) | Microsoft Docs