why can't i upgrade my ram by rooting and other functions

i can't get how to make ram more mb by ram expander that i have downloaded from play store

I'm not sure what 'ram expander' claims are, but you can't upgrade your RAM because your phone's RAM is physical memory. So unless you go into the phone and change the actual, physical RAM chip and went ahead and changed your OS's code to account for this hardware change, a simple app/rooting would not do such a thing.