If you want to display a number 999999999.99 as 999,999,999.99 then it’s normally best to do this in the front end application but if you must do it in sql the way to do it is with the money type.
You simply store your data as money or convert it to money on the fly using convert. Then wrap the value in a convert to varchar with a style of 1
DECLARE @MoneyValue float = 999999999999.99
select CONVERT (varchar, CONVERT(money,@MoneyValue ) ,1)