It can be very difficult to find them all. Very small amounts of water can host larvae as long as it is persistent. Potted plants that are watered often and outside drip lines for AC's are really common spots for unintended mosquito farms.

Unless you live on a very large property, the range of the mosquitoes usually far exceed the range of your control over standing water.

That depends on the species. Some species tend to stay within a few hundred feet of their breeding grounds, while others can travel for miles (but even then many will still tend to stay very close by).

The Aedes mosquitoes that are invading Los Angeles will apparently breed in an upturned bottle cap or "in the depression of a wrinkled old bag of chips that gets hit by a sprinkler.". Good luck finding all pools of water that big on any significant property. (LA Times semi-paywalled article: https://www.latimes.com/science/sciencenow/la-sci-sn-aedes-m... )