Selecting only the bottle's INSIDE bottom face, to form upward creating a 'hill' of glass in the bottom Looking at the picture I added to model by, you can see the bottle has a "hill" of glass in the bottom. My question is how do I 'get inside' the bottle to select only the inside bottom's top face to form upward, thereby creating a hill? Currently I have a solidify modifier added (but not applied).
Blender allows you to hide vertices for convenience in Edit Mode

Apply the Solidify modifier.

In edit mode select the vertices you do not want to edit.
Hide those vertices.  Edit the visible vertices.



Alternative.
Change to wireframe mode.  Edit mode. Select the top of the hill vertex which you can see with difficulty.  Press Control [Numeric Keypad + ] to grow your selection.  Then hide the unselected.

[Control I] can invert your selection.