Chocolate mousse - methods to have firm foamy mousse using white egg and cream
In the referenced mousse recipes (there is more than one in that dessert), the vast majority of the foaminess will come from the whipped cream.

You need to ensure that your cream is beaten properly to maximize foaminess, that is air volume:


Chill your working equipment, including the bowl, whisk, and of course the cream itself
If whipping by hand, use a large balloon whisk
Beat until holding the whipped cream up from the whisk does not cause it to lose its shape--this is the stiff peak stage.  You need to be careful, as this is the last stage before overbeaten.


In the praline mousse, the purpose of heating the eggs and sugar is so that the gelatin will dissolve; the gelatin will provide the stability for the molded dessert after it is formed and chilled.  The air beaten into the egg yolk mixture will help a little, but it is the more minor factor in the overall mousse.

In the panache based mousse, half of the cream is used to form the ganache, but again, the foam comes from the whipped cream.  In this mousse the post beating stability will come from the cocoa butter firming up when the dessert is chilled.