Large murals on, for example, commercial buildings or residences are typically commissioned. These are big enough to require scaffolding/lifts and take multiple days to paint; with some exceptions (vacant property) it'd be hard to pull that off without the owner calling the cops. The building owner is paying them for the mural, or in some cases there's city grants or arts council projects.

Lots of muralists document the art/business on youtube! Two I like: Kiptoe and SmoeNova