How to convert STL. file to CAD file?

use the CATIA Digitized Shape Editor product

https://resources.rand3d.com/insights-from-within/scanned-data-in-catia-v5-an-intro-to-digitized-shape-editor-and-quick-surface-reconstruction

My immediate thought is that "CAD" is vague. As this is in the Catia group, the previous answer is probably applicably accurate.
As an alternative, you may want to consider 3rd party applications that can load mesh based files (STL being a very common one) and convert to a boundary representation file. My company makes an application (https://instepstudio3d.com/) that does this, but you can also use other tools such as FreeCAD or Fusion to achieve this. Generally, you'll work through a process that will create an intermediate output (STEP for the InStep application) which is well supported by CAD applications, including Catia... and then you may or may not need to rebuild parts of the file.