How do they get the bubbles in aero chocolate bar?

Bubbles in chocolate (chocolate bloom) are caused by temperature fluctuations or moisture that is trapped inside the chocolate.

For an aerated chocolate bar such as Aero, nitrogen gas is injected straight into the liquid chocolate mass. Because the chocolate isn’t tempered during the foaming process, the chocolate solids end up in very small stable particles which are well dispersed in the chocolate. This process is called conching. It gives Aero chocolate its creamy texture and also results in the bubbles of air that give Aero its name.

However, bubbles in home made chocolate are due to fat bloom or sugar bloom, which is caused due to improper melting of the chocolate, incorrect storage, or adding wrong ingredients while making the chocolate.