You need to use a volumetric mesh generator, such as Cleaver or TetGen.
If you want to experiment with them without writing code, you can try them in 3D Slicer: install 3D Slicer, install Segment Mesher extension, load your STL files into Slicer as segmentation, use Segment mesher module to generate volumetric meshes, save volumetric mesh as .vtu file. You may need to tune mesh generation parameters (e.g., increase scale parameter for Cleaver to get higher-resolution mesh) and regenerate mesh until you are satisfied with mesh quality.