SketchUp has to use certain rules to figure out which way to orient the results of a pushpull. If you can figure these out and remember, fine, but it is often the case they end up backwards while you are working. You only have to select them and reverse them. This would be easier if the walls and floor in your example were in their own group.
I find that it works quite well to take a copy of the inside floor surface (no outside walls or space under the outside walls, group or make sure it is not attached to anything else). Make sure the white side is up. Now draw your walls on it. Then you have a 2d plan of white wall footprint on a white floor. Select the wall footprint and group it. Enter (“Edit”) that group and push pull the wall up. All surfaces, including the bottom will be white, from outside views. You then have the interior walls without anything else in a group, which IMO is the desired end result, prior to adding doors.
Whether the bottom is open or closed is mostly a preference for how you work or intend to use the model. If you make a “section cut” plan view and don’t use the “section fill”, you will see the bottom of the wall in the drawing. OK, but if you are running the floor surface under there too, you will get “z-flashing” and possibly variable results in the final image. On the other hand if you always use “section fill” in the plan view, it doesn’t matter. Someone is sure to come along and have a reason one way or another, but people have their different work-flows.