How I plant an Apple tree in clay.
1. Dig decent size hole.
2. Remove all media off roots. You don’t want bark and other junk rotting in the hole.
3. Spread roots outward and begin covering roots in layers. Watering as you go.
4. Mulch and add a wind break and shade cloth.
This isn’t rocket science, yes clay holds water but I root tons of plants in pure water. The issue is when you put “soil” in with the clay. Soil these days is bark, sticks, peat, etc. Not soil at all. Do not add this, it’s what rots then leads to root rot.
I have the luxury of having woods. I dig some top soil under the leaves from my gulleys from the south side. I do add a little worm castings, well finished compost, and leaf mold. Tiny amounts all you are doing is adding microbes not making “drainage”.