Just for a simple example. Take a standard Celtic Trinity Knot. I want the top (like the black plane in the red/black picture) to be a plane in the shape of a trinity knot. The bottom plane (like the red plane in the red/black picture) should be the same shape but with an extra "thickness". I need this to be a volume so I need the planes connected with side walls. The blue picture is a basic volume made of this shape. In the red and black picture it shows the different sizes of the two planes that I need. I can make the blue picture's volume but I need to know how to make one of the sides slightly "bigger" or "thicker" like shown in the red and black picture. In the red/black picture the planes are overlapped to show how the red plane is "bigger/thicker".
Select the faces along the outer and inner perimeters and press Alt+E > Extrude Faces Along Normals
While you are extruding and moving the mouse inwards, hold down Alt so you get an Even Thickness. Then click to confirm. While the new faces are highlighted, press Ctrl+Numpad+ to increase the selection and Assign the red material to it.
Here's the final result: