I installed ESP32, but can'i find my Arduino ESP32 Examples, why?
Which examples are you looking for? Examples typically come with a library, not a specific board. You select which board you are using and a library example and that code will be the proper code for the selected board, if the library supports the board.
I like to use a ESP32 CAM example.
I don't have ESP32 example at all.
I found a picture shown the example, but I don't have it in my IDE:
As blh64 said, examples are associated with libraries. However, libraries may be associated with boards. The boards platform authors may bundle a collection of libraries with the platform. This is the case with the WiFi library you are seeing in the screenshot (you can see it in the repository here).
These "platform bundled" libraries are typically written specifically for the architecture of the boards platforms. For this reason, they are only shown when you have one of the boards of that platform selected from the Arduino IDE's Tools > Board menu. So make sure you have an ESP32 board selected and then look in the File > Examples > WiFi menu again.
I selected ESP32 Dev Module board, there is no 'ESP32' for board select. The example ESP32 -- Camera -- CameraWebServer come out. There is no Camera -- WiFi appear yet.
I'm not familiar with this example. If you provide more information, we might be able to help you out with that.
The ESP32 coming out from Example when the ESP32 board selected. The content is not same as the picture attached before, I guess the reason is I installed too many Libs and the examples got reclassification.
I don't think that's it. More likely, the ESP32 boards platform developers have adjusted the examples over time. If you tell me exactly which example you're missing, I might be able to give you more information. You mention a sketch named "Camera -- WiFi", but I don't see any such sketch in the screenshot you shared, and in fact this isn't even a valid sketch name since they aren't allowed to contain spaces.