They’re making a mould of his chest with the coloured alginate or more likely a more expensive mould making material. The first picture in the description is after it’s been taken off. Robert Downey Jr has pectus excavatum, which is the opposite of having a pigeon chest which you can really see in that picture.
They’d take that to create a bust so speak to create the prosthetic on. The negative mould with probably be filled with plaster. When that’s set it’ll be used to sculpt the chest piece in clay, which again will have a mould made and then run out as a silicone prosthetic - seen in the last picture.