They are tied in wherever they would have an effect, similar to how mutations are. So for example the air purifier code probably is present anywhere that deals with breathing.
The easiest way I find to track it down is just to do a mass file search (either through snazzy stuff or through something as simple as opening up all of the various source files in N++ and then doing a “find in all files”) for the id (i.e. bio_purifier).
