Hello.

I'd like to ask about shape of nursery containers.

All of them have bottom that is not flat.

When you transplant them to the ground there is an airgap between rootball and ground soil.

What happens to the seedling roots when they don't touch ground?

I made an image.

I think you're supposed to press down on the newly planted plants after you backfill the hole to push those roots down so they WILL touch soil without an air gap. That's what I've always seen and heard on gardening videos and been doing it for 50 years that way.