How does a truck crane boom extend?

Somebody told me that there is a huge telescoping hydraulic cylinder, yet where would all the hydraulic fluid be kept?
Another person told me that there is a hydraulic motor that drives an internal chain, yet I can’t visualize this. Any explanations would be appreciated, or perhaps pointing me to a diagram. Thanks
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Is it safe to extend the truck-mounted boom concrete pump?

Is it safe to extend the truck-mounted boom concrete pump by linking additional rubber and/or steel hoses for pumping concrete to higher levels than its capacity?