All of the above is correct...
You need a valid part 121 or 135 ID and a valid pilots licence...
Some companies take you on on the jumpseat if you are a licenced pilot and you have a paid ticket and you fill a Jumpseat form.
But more usually you have to have an agreement btwn your airline and the one you want to jump with.
Captain though has the final word about who sits in the cocpit.
JJ