To be pedantic, and risking your country being different to what I've personally seen around the world, what you see attached to the wall of a building isn't a fire hydrant.

These are standpipes [0] which can be used to move water to parts of the the building from that external fitting. They may contain some static water or be completely dry (like a dry riser [1]) but they are not a source of water like a hydrant is.

[0] https://en.wikipedia.org/wiki/Standpipe_(firefighting)

[1] https://en.wikipedia.org/wiki/Dry_riser