I will have to reverse engineer from STL files ( reconstruction of CAD surfaces of Pistons, Connecting Rods, Valves...) to get STP files that can be used in Solidworks 2018, Siemens NX, Catia...
Would there be a tutorial that will explain the steps to me?
For soldiwoks: the safest method is certainly to convert your STL into PART, to check that all the surfaces connect well and that you have all the right dimensions, especially the tolerances on the diameters.
Once this step is done then your SLDPRT and STP.
I don't know why you want to switch to STP but if it's to do 3D printing you have to be very careful with all the tapping, and other precautions to take.