STL and CAD formats.
It is often very difficult for a person who has recently encountered "3d technologies" to understand all these extensions and file formats. Meanwhile, it is enough to understand a simple thing - data formats are fundamentally divided into two large groups. Moreover, automatic conversions from format to format using special converters are possible within each of the groups. But between groups, such transformations are not so simple, especially the conversion of STL to CAD.