every one of them questions has lots of answers for instance if you have heavy clay soil you ccan dig right down beside your fotting with no issues if its soft sand you may have to stay ten feet away from them
how deep depends on climate amd size depends on how much you want to store
a good start to get the ide how it works is to get a few five gallon pails with tight fitting lids bury them to the lip put what you want to sotre inside and set a
hay bail on top, tarp the hay bale if it rains a lot in winter
this may not work in very harsh winters or very hot summers but it will give you an idea what more you need to do