A bit late to the party. To answer the second part of the question

is there any other or short way to do this

There is. I am sure this explanation exists in the vasts of the SO, but I did not look for. The code below does exactly the same thing as STUFF( SELECT .. FOR XML PATH .

DECLARE @agg VARCHAR(MAX)
SELECT @agg = CONCAT(@agg + ',', name) FROM temp1
SELECT @agg

When @agg is declared, its value is NULL.
When the first row of the temp1 is fetched @agg is still NULL.
@agg + ',' equals to NULL + ',' which results in NULL. See Microsoft Documentation

However, CONCAT(NULL, Name) produces aaa, not NULL!
Then aaa is stored in @agg.

When the second row of the Temp1 is fetched CONCAT(@agg + ',', Name) produces aaa,bbb. Then this value is stored in @agg.

And so forth