The tenders, as those trucks are known, would be from the fire department themselves. Filling from nearby-as-possible ponds or tanks, a good-sized structure fire will still go through multiple trucks' worth of water.

I volunteer with our rural, island-based fire department. The more populated part has 90% or more of the people, and is part of our coverage area. The larger part is more parks and forest, and sees regional coverage such as chopper- or truck-based crews, helicopters and so on.

Each of the engine trucks carries something like 1000 gallons of water, and is kept full. So are the tenders, which can dump into the "porta-pond" as in that video. The water supply team has a list of available sources of water that they can go to to keep that water flowing, but if there is nothing nearby, the round-trip time will get longer, and firefighters will have to leave the building until there is water available.

Wildfire fighting is very different, and much more focused on containing the edges of the fire by using natural fire breaks or creating new ones. (Think picks and shovels, heli buckets, and heavy equipment where available.) If there is a house or two threatened, maybe some sprinklers can be set up ahead of the fire. If the house is on fire, too bad, efforts are on what's ahead.