There is a trick behind it, I am unsure if I can properly explain it though there are several things to take note of:
1- The texture used needs to convey an effect that matches the cell shading effect (See Boarder lands, Ni No Kini, Tell Tale - The Walking Dead, sly cooper), each comes with a unique texture style and is consistent through out.
2- The Mesh or the skeletal mesh can be duplicated, expanded and flipped to face inwards and an unlit material can be applied to it so it would give the illusion of an outline.
3- Post processing can be used to achieve two effects that might help (Outlines, Texture overlay effect, where it would look like the game has been drawn on top of a canvas).
The following steps uses a sphere as an example but it can be done with about any model you desire, Please read on:
On the modeling software
1- Create a sphere.
2- Create a slightly bigger sphere and flip the Normals of that bigger sphere (Basically you flip the faces that are facing outwards to face inwards).
3- Apply a material on the small sphere and another material on the larger sphere.
4- Export both spheres as a single object.
In Unreal Engine
1- Import the Spheres.
2- Create a simple material (Anything would do).
3- Create another material with the following option (Unlit), then add black to the emissive (you could add the color that you want)
4- Apply the first material (step 2) to the small sphere, and the unlit material (step 3) to the outer material.
5- You should now see an outline effect on the sphere.
Hope this helps