STEP (.step or .stp) stores exact CAD geometry: a cylinder is a true cylinder. Slicers such as Cura, PrusaSlicer and Bambu Studio need a mesh of triangles instead, usually STL.
What changes in the conversion
Every curved surface is tessellated into flat triangles, so round parts become faceted. The CAD feature history and dimensions do not survive — keep the STEP file as your editable master.
Check the scale
STL has no units. Slicers assume millimetres, which matches almost all mechanical CAD, but confirm the size when you import.
Assemblies
A single-file assembly is meshed into one STL. If the top STEP file only references parts stored in separate files, export a single-file STEP from your CAD tool first.