It's not that complicated (though it's more complicated than the "just bang the rocks together" actual bomb), but you do need a lot of uranium ore to get the small percentage of U235 out.
(The plutonium bomb couldn't be done with "bang the rocks together", but I'll let the physicists here explain why)
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(The plutonium bomb couldn't be done with "bang the rocks together", but I'll let the physicists here explain why)