How is uneveness handled in plastic surgery when breast implants are done. One side uneven and the other breast has lost "lift" how is surgery done?

Breast symmetry. No one is exactly symmetrical. A plastic surgeon will do his/her best to make the breasts appear as symmetrical as possible. To do so may require using implants of different sizes and/or lifting the breasts different amounts.
Depends. Sometimes different volumes of implants are required to correct asymmetries. Occasionally a mastopexy (breast lift) is needed as well. Seek consultation from an experienced board certified plastic surgeon to evaluate your needs.
Less asymm not symm. When treating uneven breasts, the goal should be to expect less asymmetry and not perfect symmetry which is near impossible to achieve. That said, if breasts have varying degrees of ptosis (sag) the differential lifts must be done; if volume is uneven, different size implants may compensate or partial reduction of the larger breast with similar implants may be used; if creases are uneven, etc...
It depends. Surgery may require using different size implants and/or employing lift techniques which may vary from one side to another. Only an in-person examination with a board-certified plastic surgeon can fully address this. Sometimes we address the two sides with different techniques, and sometimes we simply extend the technique more on one side. Most breasts are not symmetric naturally but we try...
Breast Asymmetry? Almost every patient who presents for breast surgery has some degree of breast asymmetry. Achieving absolute symmetry is usually not possible. I tell all my patients that achieving realistic expectations is important and that the breasts will be “sisters, not twins”. Sometimes the use of different sizes/profiles of breast implants plus/minus breast lifting help achieve as much symmetry as possible.
Uneven breasts. Can be made more symmetrical with implants and variations of lifts and reductions. Best to see a local plastic surgeon who can listen to your goals and examined your concerns. Options should then be provided from which you can choose from. And you are expected to be more symmetrical and but don't expect perfect.
Can be fixed. I'm assuming you mean uneveness in breast size prior to any surgery being done. Most women have some degree of difference in size and shape of their breasts. For the most part, it is usually not a problem. After breast augmentation the differences can be more noticeable and should be addressed at the time of surgery. One breast may need a different size implant or a lift to make them more equal.