Palladiotype chemistry in the traditional process is fairly simple. The coating consists of ferric oxalate, sodium chloropalladite, a small amount of oxalic acid (which is usually included in the ferric oxalate solution) and water.
Two things happen under UV light. Firstly, and most importantly, under UV the ferric oxalate is converted to ferrous oxalate. Later on (in the developer) this is used to form the palladium image.
The second reaction under UV is that a small amount of sodium chloropalladite reacts with the oxalic acid, facilitated by moisture in the coating. This lays down a small amount of pure palladium to form the faint image that is visible after exposure but before development.
The last important reaction is in the developer. The developer dissolves the ferrous oxalate which, once dissolved, is free to react with the remaining sodium chloropalladite to lay down the pure palladium image.
Platinotypes have the same reactions, just substitute potassium chloroplatinite for the sodium chloropalladite. Mixing platinum and palladium is a little more complex, but the essentials are the same.