As you must have seen in editorial, the ans is (product of (n+i) where i ranges from 1 to t+1/(t+1))-t! but

this is equal to [(product of (n+i) where i ranges from 1 to t+1)-(t+1)!]/(t+1) (multiplying t! by t+1 up and down like 2/3 +1 =(2+3)/3) this gives the explanation of first line

second line is about dividing the whole by t+1, as we have to do mod m we have to use modular division. But author has used Fermat’s little theorem details about which is given here