Nice demonstration. What did you use to capture the video?
You tutorial actually demonstrates all of the reasons you shouldn't start in 2D when working in 3D.
1. First, you should work in 3D mode - this will allow you to do things much faster with many fewer steps.
2. You should create the rectangular solid by
a. Select the "Box" tool.
b. Set the first corner anywhere (turn off the grid, it limits your freedom to work).
c. Set the diagonal opposite corner with the "Point Relative" function.
In your demo you had to do far more things, and took a lot longer, to achieve this result. There is no need to create a plane and then extrude it if you are creating rectangular boxes. It is a lot of extra work.
3. Create the cylinder with the "Cylinder" tool.
a. Select the "Cylinder" tool. Set the number of facets to 36 - do NOT use the default 144.*
b. Gravity snap to the midpoint of the lower front edge of the box.
c. Use "Point Relative" to set the desired radius.
d. Use "Point Polar" to set the length of the cylinder.
There is no need to create the circle and then extrude it. It is a lot of unnecessary work.
4. This creates the cylinder with the bottom surface coplanar with the bottom surface of the box. This can cause problems with "Solid Subtract." To be safe, move the cylinder down a slight amount.
a. Gravity snap to the cylinder to select it.
b. Gravity snap again to the same place to start the "Move" function.
c. Use "Point relative" to move the cylinder down a very short distance.
5. Use "Solid Subtract" to subtract the cylinder from the box.
* DesignCAD defaults to a huge number of facets (144) for circular planes, cylinders, etc. This will cause file size to grow rapidly, and you will soon have long redraws and huge files. In most cases 36 facets are more than enough to get smooth surfaces and ends.
The method I describe achieves the same result with FAR fewer steps, and in much less time. If you are going to do much work in 3D you should learn to use the 3D tools to maximize your productivity.
The process described above used five functions (Box, Cylinder, Midpoint, Move and Solid Subtract) and four "Point Polar" operations, a total of about 30 clicks and key presses and 48 seconds.
Your process (not counting all the shading, Info Box, etc.) used nine functions. You used "Point Polar" only once, but just dragged to set distances several times - a total of about a dozen operations. In the end you also had left over garbage (line and circular plane) that must be deleted (more operations).
Also, you messed around with the grid a lot. You should always enter precise dimensions (as I did using "Point Polar") if you are actually designing something that has to be a specific size. In such cases the grid can be a big nuisance, preventing you from placing points where they need to be.