How to make sappling?

To make trees, you need to make custom blocks for it.

Then build the tree in Minecraft and import it as structure using structure blocks.

Make the Tree as a structure element in MCreator to spawn them around the world.

To make saplings, you need to create a plant element in MCreator.

Then make it so that if the player right clicked the plant, it summons the structure of the tree in place of the sapling.

I  made a video about trees and saplings. But it is in German so maybe not the best way to solve your problem: https://www.youtube.com/watch?v=XjqMf-0gnrU