Here is a free script that demonstrates how to get a random object based on probability. I've put comments in the code to explain how the probabilities work.
Get Random Object with Probability – C# Script
How to paint random tiles with weighted probability – Unity Tilemap | 2D Extras
I have written a custom brush script that will allow you to paint random tiles based on a probability. Here is the free code script for it.
Stuart's Pixel Games 