How to write a function that automatically refers to the cell from which Excel cells are joined, if there is an expression that refers to the cell before they are joined.

There seems to be no function or setting that automatically references the value in the upper left corner of the combined cell.

Therefore, other QA suggests using MergeArea for VBA.
In addition, other QA explains how to use empty character determination and OFFSET to obtain non-empty cell values regardless of cell combination.

There is no other method where the combined されるtoto が value cannot be to 0.
As shown in the figure, the value can be left on the back of the combined cell, so other cells can have the same value as the upper left cell after the combined cell to simulate auto-reference.

1. Paste the A1 value into the A2, B1, and B2 expected binding cells (the A2 value intentionally contains a different value)
2. Prepare bound cells like Range(D1, E2)
3. Copy the combined cells and paste only the formatting on A1

Of course, if the value of A2 is incorrect, referring to the target cell in the function will result in a different value than intended, and if the value of the combined cell is corrected, the value other than A1 will not change if the B1 and B2 cells do not refer to A1.
I don't think it's a very effective method, but it can be achieved in terms of appearance.
I will leave it as an answer as it may help those who have different values when referring to the combined cells.