Banksy designs art via stencils for other people to go and get them setup.

Stencils for large outdoor art pieces has been the standard for a few decades now because you need to be quick to avoid police noticing.

It also adds a level of plausible deniability in that if Banksy or any other members of the collective get caught, it can always be played off as "We just got paid for this gig, that's all we know."