How are the original "planter beds" in atrium 'constructed'?

I want to get rid of most of the plants in my atrium, and potentially reuse some of the space which is currently "planter bed" for other uses (and perhaps just cover some of it over again with concrete even). But I'm wondering before I start digging, how are these 'constructed'? Are the planter parts just "holes" with dirt in them, eventually sitting on top of the gravel (or other substrate?) under/next to the foundation? Or is it a 'channel' with more concrete at the bottom (I suspect not given the size of the trees growing in mine), or what?

In that same vein, does anyone know, were the concrete pads in the atrium just poured after the foundation and next to it, or are they somehow part of the original pour/foundation? And has anyone had them leveled (how big a job was it?)

Thanks in advance for any advice!