How do you create under a tree that has big roots on top of ground?

Maybe stepables will work. They'll spread over top of the roots.

https://www.stepables.com/

Hard to grow plants under trees except ferns and hosta. Even tougher with surface roots. All you can do is settle for hosta and ferns. You can create a border around the tree with landscape blocks and plant a raised bed.

You could add dirt to the roots and plant in between

We had this issue with a tree in our small front yard . We bought some landscaping rock and made a large ring around the tree , making a low circular wall. Our rock wasn't the flat rocks , they were in different shape and sizes, a little harder to work with , sort of like a big puzzle, but it worked well for going around some of the roots. Then we put in a lot of top soil and covered the roots. Then I planted flowers, the area under the tree gets enough light , more on the street side than the house side. We did this years ago and it has been fine ! I even planted bulbs , daffodils and Lily of the valley !

Be careful covering the exposed roots with dirt. Some trees need exposed roots and covering them up will actually cause root rot and kill the tree from the inside out. You won’t know it’s dying until it falls or a wind storm causes a break. Check out the tree species before covering the roots. Planting hostas and ferns may be the best bet.

It's tough to plant under a tree with exposed roots because if you cover them it may suffocate the tree. I read somewhere that trees need lots of water or their roots will grow too close to the surface in order to get water. Once the roots have been exposed, it's too late to fix it however, planting in between the roots with hostas and ferns sounds like a good option. Once they mature they will cover the roots without suffocating them.

Thank you everyone!!!

Creeping Myrtle or Periwinkle. This famous Vinca spreads to make great patches under trees. Blue flowers. (Vinca minor)

Make sure you place landscape edging out from the tree it will fill in over several years and cover the entire radius of the tree. It allows moisture into the tree roots and does not require special attention. Beautiful purple blue flowers in spring with shiny small oval green leaves over summer and fall.

In my case, the old day lilly is thriving at an invasive rate. But it does fill in between the roots well.