Noob modeling question

Hi, I am new to Blender for modeling, have moderate Solidworks experience and am trying to modify a STL and do not know functions to get to my end goal. I am looking at making a cavity in a flat surface that will be the exact geometry of the rounded item. This will create a nest point so that I can 3D print both separately and be able to place one item into the other to hold it. I am just starting work on this now and files are in STL format.

Also, if I want to manual augment the surface of a STL object to add say a vein, ridges, or extra surface contour, how can this be done? This is for a Sci Fi model, have image in my head just having issue getting finished product onto computer.

Is this like mold and casting, where surface feature is copied from mold to casting?

If so, all you need to define is the surface feature in 3D. Then use that surface to create inside object and outside object. It is just a matter of adding volume to defined surface on the outside for one and inside for other.

Simple way to do this with blender is to use Solidify modifier to create two object using one formed face. Just play with thickness setting; one positive and one negative.

Thank you for feedback on this, let me give that a try. I eventually want to start trying to make molds but for now am just trying to create a simple formed cradle for an item using the negative of its shape so that it is a good clean fit.