There are two methods to see inside a building, each with different features.
First, you can press the N key. This uses cutbox mode so as you close in on a building it will cut away part of the building.
The second option is using the slider bar. Look on the right of the screen for the green diamond. You can drag that down and if you are viewing close enough to the building that will take away any layers above the set height of the bar. If your view is far away, it won’t have an impact so make sure you’re close enough to see when it starts hiding the upper layers.