There are many different 3D design softwares, with only minor differences between their core functions. Most differences only arise because the programs are developed by different companies. A few are free and open-source programs. most have license arrangements that allow free student or hobby use, with a public account.
Fusion 360
Inventor
SOLIDWORKS
OnShape
Unreal Engine
Blender
Antimony
OpenSCAD
There are some basic skills that are needed with all graphical 3D design software:
Understanding the filetypes for parts
The file type for a single part
The file type for 'assemblies' of multiple parts together
The STL file type for storing geometry only (used for 3D printing)
Drawing 2D sketches before extrusion or cutting
Sketches can be drawn on a starting plane
Sketches can be drawn on existing flat surfaces of the model
Sketches can be drawn on insterted planes, placed in/on the part by the user
Extrusion and cutting of the model, from a sketch
Extrusion adds material to the model
Cutting removes material, as the opposite of an extrusion
*All shapes on the sketch need to be 'closed' in order to cut or extrude
*Extrusions and cuts have options to move away from their originating sketch in various amounts and directions
*There are many more skills around the basic Extrusion and Cut actions
There are advanced options to edit 3D models that often vary per software
Linear Arrays to expand a grid of model properties
Circular Arrays to expand a circular pattern of model properties
Loft to extrude in a nonlinear direction
Shell to remove a side and hollow out a shape (to a specified thickness)
Bevel/ Chambfer to curve or angle the edges of a shape
*There are usually many more advanced options, but these vary with each type of software...
Dimensioned design allows you to build to specifications
The dimension tool allows you to specify sizes in sketches and extrusions
Constraints allow you to set relationships between parts of a design
3D designs can be made to relate to real-world materials and objects to the model
Parameter tables can allow dimensions to have names, such as "thickness"
Parametric design allows you to build to specifications, with an ability to scale your work
A good parametric design includes relationships between features of an object
Sketch elements will have constraints that hold their relationships
Dimensions will be related via functions in parameter table for scalability [ex: width = 5 * thickness]
Assemblies of parts may be interrelated, and designed to fit together
