Hello Bálint Papp
I reviewed the model and below are some pointers to convert the surface geometry to solid geometry:
1. The body named "San+Francisco Geometry" represents most of the geometry. There are a few additional surface bodies in the structure tree. So hide the San Fransisco surface body and use the Repair tools to convert these smaller surface bodies into solid bodies.
2. If you look at the San Fransisco geometry from bottom and highlight the ground face, you can see imprints of most of the bodies. See below snapshot. Right click on this face and select "Detach" option. This will not only detach the ground surface but also convert all the "closed" surface bodies into solid bodies.
3. Once done, select any of the solid bodies created and using Power selection, select all the solid bodies and hide them. This will display only the remaining surface bodies that you need to work on. For convenience, hide the ground surface and the adjacent 2 surface bodies shown below to display the buildings only.
4. Many of the surface bodies which you can now see have hanging faces. See below snapshots.
You will have to review each of these bodies and perform operations to fix these hanging surfaces i.e either delete them, or create additional faces to form a closed region. I would suggest displaying one surface body at a time and then performing the 3 repair tools (Stitch, Gas, Missing faces). This will help to identify the regions to fix. The 3 repair tools won't always help to fix the identified problems. For example, the missing faces tool might help to identify the hanging faces but they might not get fixed by the tool. You will have to use other operations like delete, or create new faces, etc. The final aim should be to create a single closed region for each of these surface bodies so that they get converted to solid bodies.
On similar lines, you can work on remaining surface bodies and convert them into solid bodies.
Please let me know if you have any questions or need further assistance.