As explained in the direct by Starfield's lead technical producer, Jean-Francois Levesque, the game's system "builds the planet as the player approaches it." The game seemingly specks the planet with different types of "hand-crafted content," and it does so randomly and dynamically, so while you may find a questline in one planet, a friend of yours may find it in another.
When it comes to what type of content the planets will have, the direct mentioned different resources for players to gather, outposts, plants, and creatures, both peaceful and hostile. As previously mentioned, the procedural generation will also include questlines, meaning some side quests will be in different places in the world map per save file.
There will, of course, be some static planets that presumably don't change as they're particularly relevant to the story, or they've simply been purposefully hand-crafted in their entirety. A few of these were mentioned in the direct, such as Alpha Centauri, the hub of Starfield's central civilized faction (the UC), Mars, which houses the UC's largest mining facility, and Volii, home of Neon, the pleasure city, among many others.