difference between grep Vs cat and grep

Question

i would like to know difference between below 2 commands, I understand that 2) should be use but i want to know the exact sequence that happens in 1) and 2) suppose filename

Answer 1

Actually, though the outputs are the same;

-$cat filename | grep regex

This command looks for the content of the file "filename", then fetches regex in it; while

-$grep regex filename

This command directly searches for the content named regex in the file "filename"