The offset function, as I have used it, is broken down to:
Offset(from the anchor, move this many rows, and this many columns)
Sheet1!$B$21 is the anchor
COLUMN()-3 is the number of rows to move. You gave C10 as start cell or Column 3 - Row 10. As you copy the formula to the right, this number will grow forcing the "move this many rows" to increase in the Offset.
Row()-10 is the same principal for determining the number of columns to move from the anchor.
Using the Index function:
=INDEX(Sheet1!$B$21:$E$28,COLUMN()-2,ROW()-9)
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