i need help with my custom villager

hello,im currently creating a mod around ores and farming and im making a custom entity (that works as a normal villager) i have all of the textures,behavior and stuff but i dont know how to make the gui exactly like it is in normal minecraft and i would need the trades to include my custom items and for the trades to be random, like in normal minecraft each villager has different trades.example, you find 2 farmers,1 buys carrots for emerald and the other buys potatoes for emeralds. i want this system to work on my custom villager but i dont know how. can someone help me with it?