The inside of the structure is quite empty. You will find a trader, a PC, 6 healers and a nice little trainer relaxation area. The center is lit by glowstone on the ceiling which are hanging by fence posts. There are also redstone lamps in the flooring, however they need a signal to be activated.
The Jungle variant is one story with a basement and has lots of cobble and mossy cobble in its design. It is made to look old and overgrown
The Pokémon Center seems to most common in a plains biome and rare in the other biomes. This is likely because it could use a similar code to the village spawns where they need more flat land. The jungle varient appears sunk into the ground, even cutting trees in half, implying that this variant uses different code.