just before I found indication how split geometry
Here the link: How to Split an STL geometry into different faces
This operation is tipicaly if for users that use STL.
At least I was not able to split surfaces with simscale, here the notice after several trials
The STL file contains at least one edge where three triangles join (it describes a non-manifold surface). This might lead to issues in the downstream workflow (e.g. splitting in separate faces will not be possible, mesh generation might fail). To be safe, upload an STL in which at most two triangles share one edge.
I think Archicad is not the best option for SimScale but it is my only option to realize 3d models