most guides are freelance and are hired for particular jobs.
Tour operators and other people employ guides mainly to inform tourists about the places they are visiting.
Therefore a guide has to have a good sound knowledge not only of a particular place but also of other things which are generally relevant - for example, architecture, history, and local customs.
During our training we intensively learn a vast amount of information about a whole range of subjects, and we have to be capable of jumping from one topic to another in the same sentencel ! But the way in which a person conveys this knowledge is the key: you have to be good at judging what your audience is interested in and you have to know how to keep their attention.
These are not easy skills, I can tell you!