Have a look at https://developers.google.com/earth-engine/classification

https://geohackweek.github.io/GoogleEarthEngine/05-classify-imagery/

I want to know what should put in the argument as treestring

You can see a practical example for your question via this link:

https://datasciencecastnet.home.blog/tag/gis/

The link for the generating and the exporting of the decision tree is missing in this blog

Unfortunately, I didn't use it. I Test classification methods for internet network traffic