The class-A amplifier is too simple to show. The article assumes that the reader knows a little about a basic transistor class-A amplifier.
The article has an example circuit of a Practical Class-AB Power Amplifier, like is used in many opamps and audio ICs.
A class-C amplifier is full of distortion so is never used for audio, just for RF where there is a tuned circuit and filters to remove the interfering distortion, and you must know RF to use it.
A class-D or class-H amplifier is too complicated to show in an article that describes them. You can find ICs that do it.
I know how a basic class a works. But for the others you need to identify and/or build them, you need to see the component placement and how they look on schematic.
A class-C amplifier is full of distortion so is never used for audio, just for RF where there is a tuned circuit and filters to remove the interfering distortion, and you must know RF to use it.