There is no 1, difinitive, original Paella. The varieties of the Spanish rice casserole are as numerous as the imagination will allow.
The main secrets are in the kind of rice you use and the method. To make genuine paella, you should look for Valencia rice, named for the region of Spain where it originates. Secondly, the method has a lot in common with Italian risotto - thought the pan is different.
One more secret is don't cook the meats in the paella for the entire time that the rice is cooking. Almost anything you choose (fowl, fish, shellfish, game, sausage, etc.) will be quite overcooked by the time the rice is done unless you remove it for most of the rice cooking time
Having saia all this, is there a specific paella you are seeking?
Paella Valenciana is probably the closest to an "original" recipe as it originated there. If I can remember, the authentic ingredients used at that time were rabbit, chicken, lima beans, and snails along with the saffron and rice. The seafood version, Paella Andulusia, was made with an assortment of seafoood.
Originally, paella is cooked on an open fire in a classic paella pan sans cover. Usually cooked by a man -not unlike our american barbeques- showing off to his fishing buddies. The cooking method was all about controlling of the fire so as not to burn it, and the precise timing of adding the ingredients so everything would finish cooking at the same time. I hope this helps.
To add a different POV:
At the restaurant I work at we cook the rice about 75% of the way before hand with saffron and herbs, and sheet pan it to cool down.
(I'm a little sketchy on this part since it's not on my station)
When we're ready to go with the dish, we sautee some shallots and garlic, add the main ingredients (first the protiens spanish chorizo and chicken or shellfish, then the veggies (artichoke hearts, tomatoes, rosted red peppers, fava beans)). Then add the rice, hit it with some saffron stock, and cook till done.
We don't make our's with the crust, but you can easly attain on by settien on the heat for a while longer.