Import
Most CAD software allows to export meshes of parts as individual files in a common coordinate system. This means that the origin of all objects rests at a common origin and only the vertices of the meshes are placed such that when all objects are overlayed at their common origin, the original CAD part is assembled. This allows to simply import all such exported meshes in Blender and end up with correctly placed objects.
The meshes are placed in correct relation to one another when exported as a group from CAD, however each part's origin is at the global origin of Blender.
The origins of the individual parts can then be moved to the individual objects' center of mass (COM), center of geometry etc.:
Moving the origins of the objects to their approximated COM.
Now every part has it's own origin, which will be important for adding collision objects etc.
The COM thus calculated assumes a homogenous mass distribution throughout the volume of a mesh, which will rarely be the case for real robots. To obtain a precise model, you will therefore therefore have to adjust the COM of the robot's parts.
Another possibility is to export every part from CAD with the mesh's origin at the actual center of mass of that part (for which it is necessary to create coordinate systems in CAD residing at the COM). This however leads to the problem of then placing the individually exported parts correctly with respect to one another in Blender. There currently is no 'one size fits all' solution to this issue and every robot project team will have to figure out the most convenient way for them.
There are some solutions for exporting a robot from CAD not only in meshes, but as a URDF or different kinematic description. If such an export is available to you, it may be far more useful than importing raw mesh data.
Either way, you can import the meshes of your choice by clicking File -> Import -> format, with 'format' being whatever mesh format you chose for export (we recommend stl.
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