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

First one:

cat filename | grep regex

Normally cat opens file and prints its contents line by line to stdout. But here it outputs its content to pipe'|'. After that grep reads from pipe(it takes pipe as stdin) then if matches regex prints line to stdout. But here there is a detail grep is opened in new shell process so pipe forwards its input as output to new shell process.

Second one:

grep regex filename

Here grep directly reads from file(above it was reading from pipe) and matches regex if matched prints line to stdout.