Also answered here: How does solder mask layer work?, and take heed of the warnings about manufacturers and communication! Even in clear cases they may ask you again if you don’t communicate your intent explicitly and clearly the first time.
But the mask opening (maskless board) alone doesn’t mean you get “solder tinning” on the copper. You have to ask for certain kind of plating. Again, it depends on the manufacturer. Gold ENIG is cooler than tin IMO.