New Guy with Questions

It's not a beast, it's a diva .. it'll give you back a lot of pleasure if you know how to handle 😆

Hard to guess what exactly you did up to now, but here we go:

First of all buying WinPC NC is only part of the whole picture. This piece of software is only running the machine and it needs input so that it knows what to do. But let's start from the beginning.

First program in the software toolchain is the CAD (Computer Aided Design). With the CAD you "design" or "construct" the parts. Depending on the kind of part (2D, 3D, precise, artwork ...) you want to produce, you have to choose the right software. Some say that 90% of the mechanical parts come along if constructed with 2D software and being cut in 2D (simple cut through the material) or 2,5D (different heights but no curved surfaces).

So, each program that can produce for example HPGL-files or DXF-files (maybe some more formats) can be used for 2D parts.
Programs like Inkscape, CorelDraw or other vector-drawing software have nice functions for producing artwork (for example how to arrange words in any available font in circles, or give good support to convert a foto into a drawing ...) but are a little weak if it comes to precise construction.

Programs like QCAD, LibreCAD, solid edge are better candidates for constructing parts with precise dimensions.

Output of all the CAD programs are files which precisely describe the shape of the part. But if the router exactly follows the lines of these drawings, you will see that the parts produced will be smaller than planned (the outbounds) while holes would be too big (inbounds). This is because the center of your cutting tool would be following those lines, but the cutting tools have a diameter and the cutting edge has a distance r to the center.

This is why you need the next kind of software in the toolchain, the CAM (Computer Aided Manufacturing). With the CAM you load the files produced by the CAD stage and prepare them for the production. It is meant for creating the so called toolpath by taking a radius offset into account. Additionally you have to plan which tools to use and in which order to do what.
For example: if you have a part with a lot of holes it probably makes sense to drill the holes instead of cutting, because drilling is more precise, maybe faster and cutters are not really made for going straight into the material. If you only have one hole it does not make sense to add a tool-change to the production.
If you have overlapping pockets in different heights it might make sense to cut in a special order ...

Examples of available CAM software:
BoCNC, EstlCAM (very widely used here in the forum), CAMBam, Vectrix Cut2D, FilouNC12 ....

These create g-code files. The g-code in general is so to say the machine language of the CNC driven machines. It describes the toolpath for routers, plasma cutters, 3D printers, lathes .... (of course you need different CAM software for some of the machines to produce the right g-code)

The g-code is what you load with the software that controls the machine (WinPC NC in your case).
The main thing you do with this software is to set the origin of the workpiece coordinate system and run the g-code.

In general you need the same toolchain (CAD/CAM/controller) for creating 3D pieces, only the software might be different for the first 2 stages:
CAD: Fusion 360, Sparcfun mechanical, Sketchup ....
CAM: EstlCAM, Cut3D, DeskProto 3D .... (for 3D printers the CAM is called Slicer)

g-code is prepared for 3D and can even handle more axles than we have on the steppi. WinPC-NC full version can also handle a rotary axle.