When I started gardening about 50 years ago (I started very young!), the recommendation for planting was to dig a hole deeper than the root ball so you could add a nice, thick layer of good soil to the bottom of the hole… and that bit of false information is still being repeated to this day. In fact, however, studies on plant growth show that plants do better when their root ball is set on a solid base, not on fresh or loosened soil. So the best advice for planting is to dig a hole wider than the root ball, but no deeper than it is high.
The problem with amending the soil under the plant is that such loosened soil tends to compact downwards over time, with the result that root ball drops deeper (or starts to slant if the ground settles unevenly). If you make a hole only as deep as the root ball is high, though, the plant will not move, but will stay put… and you really don’t want a plant to move when it’s supposed be settling in and producing new roots.
But what about adding good soil to the planting hole? There is no need to put it under the plant: fill in around it if you feel it needs better soil. Better yet, mulch all around the plant, out to 3 times the width of the original root ball, with a rich organic mulch like chopped leaves. As the mulch decomposes, it will supply all the nutrients the plant needs.