Best way to put commas into large numbers
TSQL's FORMAT function can do that.  EG

select format(1232131312342132123.2222,'0,###.##')


outputs

1,232,131,312,342,132,123.22


If you want to use a different separator, you still use , which has a special meaning in .Net format string, and then replace the , with something else.  EG:

select replace(format(123445678.2222,'#,###.##'),',','_')


outputs

123_445_678.22