Answer: Based on your description I am not certain what is happening to your trees. The evergreen magnolias normally drop their older leaves, especially about now then they bloom, as the new growth comes on.
The leaf litter should be left in place under the tree. It rots down over time and helps feed the soil naturally. You could mulch lightly under the tree with an organic mulch about two inches thick, spread it out in a flat layer and do not allow it to touch the trunk of the tree.
You could also top dress in the spring each year with good quality compost and/or a general purpose granular fertilizer or a slow release granular fertilizer. Read and follow the label directions for how much to use.
To hide the mess caused by the constant leaf drop, keep the lower branches intact so they sweep the ground. They shade the ground and keep the soil a bit cooler and moister in the summer. This helps keep the tree healthier over time. Pruning is not recommended on these trees unless truly necessary such as due to storm breakage -- the cuts provide an invitation to disease.
The tree shades itself to some extent, thus leading to somewhat thinner foliage to the interior of the tree and toward the bottom as a natural result. But normally the bottom branches would not be entirely bare, there should be some leaves toward the ends of the branches at least. If they are completely bare, there might be a pest problem at work.
For this reason I would suggest you consult with your local county extension and/or with a professionally trained and certified arborist with experience working on ornamental trees. I'm sorry I can't be more specific for you long distance.
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