I'm on 1.20.1 and was wondering if there was a way to add a mob I am going to make to a other mod's biome (Regions Unexplored in this case). I am trying to figure this out before I start coding the mod. Could someone point me to a good guide on this? I saw some things about biome modifier's but all the resources are either to short, unhelpful or for a different version. Thanks in advance. (i am a beginner but i have java and modding knowledge so this isn't like a "i am new and don't know anything about coding" problem btw)

You can add mob spawns using Forge's Biome Modifiers feature. I won't go into details on how to use a data generator for them, but here's a simple example:

{
  "type": "forge:add_spawns", // Required
  "biomes": "#namespace:biome_tag", // Accepts a biome id, [list of biome ids], or #namespace:biome_tag
  "spawners": {
    "type": "namespace:entity_type", // Type of mob to spawn
    "weight": 100, // int, spawn weighting
    "minCount": 1, // int, minimum pack size
    "maxCount": 4 // int, maximum pack size
  }
}

This JSON file would go somewhere like this: data/modid/forge/biome_modifier/your_biome_modifier.json

tysm