During aluminum PCB manufacturing, generating the circuit on the copper layers is the same as generating an inner circuit layer on other PCBs, We place a film drawing the circuit image above the copper layer that is covered by photoresist liquid.
Under the UV light, the photoresist under the circuit image hardens and forms a protective film. While photoresist in the rest areas is still liquid and is washed away.
And then we use sodium hydroxide to etch away exposed copper. In this way, only copper in the circuit image remains. And then we peel off the photoresist film and get the required copper circuit traces.