To insert formula in specific cell with VBA

If I have a speadsheet with two colums of data. Each column has data of 1000 rows (could vary in length). If I want to insert formula in specifc cells (see below++) within the two columns, what would VBA code be?

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1. Blank cells
2. Cells with certain value, suach as "0", "XYZ"
3. When the value of other cells, let say, the cell in the same row in column A, is "0", start with "XZY", equal to "XZY", or end with "XZY".

For instance, cells B3:B20 are filled in, B21 is blank, B22:B70 are filled in, B71's value is "0", e.t.c.

Thanks.