The answers to this question provide a good overview of how a single lipo cell is charged:
Most chargers that handle lipo packs have a balance mode that ensures each cell is charged to the proper maximum voltage.
How do chargers typically handle this? Do they apply different voltages to each cell, or do they discharge cells with higher voltages until they match the voltages of the less charged cells? What kind of circuit would be used for the discharge phase (if any)?