Create a selection of all Bodies in Mechanical

all_bodies = []
for assembly in ExtAPI.DataModel.GeoData.Assemblies:
    for part in assembly.Parts:
        for body in part.Bodies:
            all_bodies.append(body.Id)
 
sel = ExtAPI.SelectionManager.CreateSelectionInfo(SelectionTypeEnum.GeometryEntities)
sel.Ids = all_bodies