Stylized graphics can best be described as an art style which is between realistic- and cartoonish graphics. A game that's using stylized graphics is the newly released Sea of Thieves. One of the developers of the game gave a GDC talk where he explained how they did it: Visual Adventures on Sea of Thieves.
Simplify structures. The graphics should not be realistic, so you have to simplify everything. But you can still achieve a complex scene if you combine several simple objects.
Paint the textures. You can't use photos as textures, so you have to try to simplify the photo reference by hand.
Notice that the textures above were not used in the game because the final textures were actually more detailed: the palm leaves for example look more like real leaves but are still simplified.
Use the light source to change the mode. An often forgotten parameter you can change is the light source in the scene.
The stuff you add should look like it has been used. Almost no products in real life comes shining straight from the factory. You have to add wear and tear: someone has hit rock with that shovel below. You also have to add patches and repair: someone has repaired that gun below with a leather strap.
To achieve this look you have to learn the word "wonky."
But you still have to make sure the products are realistically wonky.
Fortnite
Another game which has stylized graphics is Fortnite, and they have also been to GDC. Their talk is called Developing the Art of Fortnite.
Simplify the textures. Fortnite tried to use the original realistic textures and simplify them in Photoshop to achieve the style they wanted.
But they realized it was better to hand-paint the textures to give them more character because simplifying a realistic texture by using Photoshop tools made the texture too noisy.
The stuff you add should be deformed. While Sea of Thieves argued that their models should be realistically deformed, Fortnite prefers another style which is more cartoonish.
To achieve this look you just have to remove all parallel lines from a real-world object. This means you can also use photo references instead of drawing concept art.
Even though the graphics is simplified, you can still use normal- and specular maps.
The stuff you add should look like it has been used. As in Sea of Thieves, Fortnite thought it was important to add wear and tear, dust, and leaky bolts, that gives the assets a character.
Don't add anything smaller than a mailbox. Smaller objects will just slow down the player and disrupt the flow. But you can add grass which the player can't collide with and just move through the grass as if it was hollow.
