How to How to extract geometric boundaries for SD

_Geant4 Version:_11.1.2
_Operating System:_Win11
_Compiler/Version:_Visual Studio Community 2022 (17.7.0)
_CMake Version:_3.27.2

I use CADMesh to import complex structure into my simulation. Recently, I want to add a function - radiation dose/intensity distribution in a plane or within a volume- using sensitive detector. In my scheme, I need to first extract the geometric boundary of the target, then build an array of detectors with a small volume within this boundary.

Here I met a problem that I don’t know how to get the position (to locate and place the SD array) and the boundary information of geometry (.stl) imported using CADMesh.

Please give me some advice on it or if there are any better scheme which can achieve my goal.
Thank you in advance if any help is given.