In the third novel, Mockingjay, we see a window into how the Hunger Games are operated. Events are triggered by devices called pods which have been placed ahead of time, and can be activated by triggers. While the movie might at times appear to show the Hunger Games as running on almost holodeck-level technology, every time we see a more solid description of what things are or how they work, it is much more mundane.
Setting a fire to make an area inaccessible or to drive the tributes into conflict is a very normal requirement in the Hunger Games. We can reasonably infer that almost every area within an arena has some kind of area-denial pod built in, just in case.
As for the muttations, they were likely prepared in advance as a grand finale, not pulled from thin air. All of the other muttations were explicitly called out as being real genetically-modified animals developed in the war, not holographic. Holograms in this universe have always been shown as incorporeal and obvious, not physically tangible and utterly seamless. It is much more reasonable that they were made from the established line of technology, rather than another we haven't seen.
The tributes enter through trap doors, leave on hovercrafts, and see actual cameras embedded in the surroundings. The hunger game dome is a modern Colosseum, and despite all the fancy sleight of hand, it's still mostly trap doors and caged animals.