Both of the trees, the Dogwood & Magnolia have a multitude of shallow feeder roots that will have to be dealt with.
Removal of too many of those roots can jeopardize the health of those trees, so adding underplantings should be limited.
If possible, select plants with shallow root systems, such as Azaleas, Rhododendrons, Ferns, etc.
Dig a plant*ng hole at least twice as large as the existing rootball of the added pl*nt, Clean the severed roots from the soil removed and use as backfill when installing the new pl*nt. Do not add amendments or fertiliz*r to the plant*ng hole.
Until the new pl*nt(s) become well established, use a very sharp shovel each year to prune any tr*e roots that may have advanced into the original pl*nting hole.
Once the additions have become established, they will be better equipped to compete with the tr*e roots for moisture and nutrients.
If you f*rtilize or irrigate the new pl*nts, you must do so to the entire area under the tr*e's canopies, otherwise their roots will seek out the area with the added nutrients and moisture and tend to strangle the underplantings.

BTW, the Eastern Flowering Dogwood, Cornus florida, is a short lived tr*e, with average life span of about 40± years. Since yours is in it's waning years, removal of too many of it's feeder roots at one time, IMO, could hasten it's demise.

I killed a dogwood tree once by planting perennials under it that need alot of maintenance, division, etc. I have read that when you underplant trees, you will do best with small plants whose root systems won't require large holes. And, if you come upon sizeable tree roots as you are digging in the underplantings, you should not cut the roots, but find another spot. It is also recommended that you use liquid fertilizers rather than granular to avoid the trees sucking the nutrition away from the underplantings. That being said, I have a number of azaleas that share space with trees and they do fine. I would image that georgia is right about the shallow root systems. Just remember that azaleas must have consistent moisture, especially their first two years.

Thanks, Georgia-Rose and Donnabaskets. Very helpful! lily_g

I've got a shaded moist area under my Dogwood that I cherish and up untll now I'm visualizing a underplanted filler perennial like Lily -of The Valley. for interest. Georgia -Rose, I'll ask you if they're a bit to aggressive for the Dogwood root system. I don't want them to clash !! Thanks

Will hostas do well planted around dogwoods both kousa and flowering?