How do solutions act as solvents?

For example:



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> Silver chloride, $\ce{AgCl}$, dissolves in dilute aqueous ammonia, so this can be used in qualitative analysis, after reacting $\ce{Ag+}$ ions with $\ce{Cl-}$ ions, to differentiate the white chloride-containing precipitate from a similarly colored cream bromide-containing precipitate ($\ce{AgBr}$) or a yellow iodide-containing precipitate ($\ce{AgI}$).
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The ammonia is already dissolved in a solvent (presumably water), therefore the ammonia molecules are sparse and separated in the solution (because low concentration, hence "dilute"), how does the $\ce{AgCl}$ dissolve in an already dissolved solution - where the ammonia is not concentrated-in-one-space-enough to have something 'dissolve' in it (shouldn't it dissolve in the original solvent instead?)


Or do the polarities of the molecules (eg. $\ce{H2O}$ and $\ce{NH3}$) in the new solvent 'combine' somehow to make an even more polar solvent and therefore overcome the dipoles in the polar solute, making it dissolve? If this were case, why is more concentrated ammonia needed to dissolve $\ce{AgBr}$ (less polar than $\ce{AgCl}$ - hence shouldn't need as much concentration of ammonia (extra polarity) to dissolve it)? How do polarities of molecules in solvents combine?


