I'm interested what is the purpose these polygon array on below picture. Thermal management? Thanks for the help!
It's called "copper thieving". The copper is added to areas with low density of copper to create a more even distribution across the board. Even distribution helps ensure that the plating thickness is fairly even, and you don't get areas with little copper and very thick plating, and areas with heavy copper density and very thin or even missing plating. This can also affect hole plating for critical areas such as press-fit connectors.
The added copper is typically added by the PCB manufacturer, in collaboration with the customer, obviously, since adding copper in otherwise blank areas may violate some other intent of the designer. Altium comments on it. Here is a useful blog entry with design tips for avoiding problems when the fabricator adds thieving to outer layers.
Edit: Typical note to fabricator:
Those copper circles are on the inner layers and are called flow dots. In large areas with no copper the epoxy doesn't flow and fill evenly and you end up with not quite flat areas of surfaces across the PCB.
Copper thieving is generally done on the outside edges of the panel where the current density is the highest to promote more even plating of interior area of panel.