Speaking of gravity, if the earth is many many different layers of various materials, compounds and elements, where exactly does stuff gravitate to? Can the earth be considered a "whole?"
While the earth is made up of a number of layers, they are arranged reasonably concentrically.
And yes, to
precisely calculate the gravitational attraction between two objects, the sum of the attractions between each element of one and each each element of the other needs to be computed. In the general case that is quite a complicated integral. But a point outside any object with "spherical symmetry" we can lump all of its mass at its centre, hence my "reasonably concentrically" note above.
So to a
very good approximation all the mass of the earth can be taken at its "centre of mass".
For complicated objects (that are not too large) on the earth (like people etc) the distance from the parts of the object to the "centre of mass" of the earth varies so little that we can take the force as applying from their "centre of mass".
So put simply for most applications "the earth" can "be considered a whole". There are exceptions to this when very precise gravimetric measurements are being done, for example in the search for minerals found in massive deposits.
If gravitation is the direct magnetic-like force between any two molecules, then why should any molecule travel downward torwards the core of the globe? Are we drawn to the center of the mass as gravity says? How do you determine the center of an amalgamation of matter?
The only extra I could add here is that while "any molecule" might be attracted towards the "centre of mass" of the earth, it can't necessarity "travel" there because there are things in the way. The ground is solid enough to stop us moving down, even though gravitation is "pulling us".
Remember that gravitation has been measured between relatively small (compared to the earth) objects numerous times from Cavendish on, and with quite consistent results. Also we see the expected variation with altitude, latitude and the proximity to massive objects.
Any other explanation needs to explain these variations - the "Flat Earth UA" does not!
If they say gravity is the attraction to the sum of the matter beneath our feet, i.e. the other side of the globe included, that concept would be destroyed by any understanding of fluid dynamics.
I don't see where "fluid dynamics" comes into it. If you mean that the oceans sould be attracted too - well they are, and the surface of the ocean does conform to the expected shape. They can't "fall further" because there is the solid ocean floor below them. Some water probably does "leak". If from lakes and rivers this forms our "ground water" (springs, wells and bores) and in general ends up coming back up as steam from volcanoes and geysers.
Maybe you need to expand more on what you meant here.