Make an existing Sketchup model solid

The task is to make an existing Sketchup model into a solid group. It is a model of a house including the interior. The whole model, including the interior should be solid. The model is downloaded from 3d warehouse. This is the link: https://3dwarehouse.sketchup.com/model/6256f77f-d8f6-4853-a0a1-cfa6c29bcd40/Hopital-Decroze-B%C3%A2timent-Administratif

I use Sketchup 2021.