The final products are mostly the POS / Sov Structures and POS Fuel.
Therefore, it is possible to buy P3 products on the market, haul them to a planet and then process them into P4 products to be hauled back to a station and sold on the market.
Or it is possible to buy P2 products on the market, haul them to a planet and then process them into P3 products to be hauled back to a station and sold on the market.
In other words, you can make ISK from PI without actually extracting Resources from any Planet. Think of PI as Mining and Manufacturing but on a Planet.
Many claim that PI is "passive income" - mainly because you can set up facilities on a planet to automatically extract resources + combine them to make P1 products + combine them to make P2 products all on the same planet and all this occurs when you are offline, much like manufacturing. This can be set up to happen over a week and all the player needs to do is adjust the extractors once a week (need not even undock to do that) and turn up to the planet once a month to take the finished products to the market for sale. Sort of like mining AFK.
I will be going more into what i am doing in later posts but i note that the likes of Foo who blogs at foo-eve.blogspot makes part of his living with Planetary Interaction in Wormhole space.
The 8 minute CCP Planetary Interaction guide is a good place to start.
Two 40 minute videos by Gorthour Decievers on Planetary Interactions. The first video deals with setting up an extraction Planet. The second video deals with setting up a factory planet. Excellent starting videos for Planetary Interaction.
They are included on my useful video page.