virtualbox.org

Is it possible to choose a setting for RAM so that virtualbox chooses dynamic RAM instead of the fixed RAM? So that virtualbox can choose how much ram it needs instead of me giving it dedicated amount of ram?

AFAIR no, there is no feature like that. There is a feature called memory ballooning which gives memory pages back to the host if the guest doesn't need them, but that memory is only available to other VirtualBox VMs which also enable memory ballooning. See user manual for details.

Besides, a PC generally uses whatever RAM is available. It doesn't have a concept of how much RAM it needs. Also VirtualBox has no access to the memory allocator inside the guest OS, so VirtualBox doesn't know how full the RAM is. Yes it could (and does, via ballooning) track when memory pages have been written to, but it can't, for example, know when memory has been deallocated.

IMHO, it's the other way around:

There is a VirtualBox setting to force the pre-allocation of the RAM before the guest is started, and by default it's not set. So the default is lazy allocation of RAM. But: Some OSs (e.g. from the Microsoft Windows family of OSs) touch all available RAM very early and therefore cancel out the lazy allocation done by VirtualBox.