In Kotlin (build.gradle.kts)

repositories {
    maven("https://api.modrinth.com/maven")
}

dependencies {
    // Replace [VERSION] with whatever latest version is available 
    // for the Minecraft version and loader you need
    modImplementation("maven.modrinth:millies-config-n-utils:[VERSION]")
}

In Groovy (build.gradle)

I dunno

package net.yourmod.mod;

import net.justmili.libs.config.FileType;
import net.justmili.libs.config.build.ConfigEntry;
import net.justmili.libs.config.build.ListConfigEntry;
import net.justmili.libs.config.build.MConfigBuilder;

import java.util.List;

public class ExampleConfig {
    public static ConfigEntry<Integer> someInt;
    public static ConfigEntry<Float> someFloat;
    public static ConfigEntry<Double> someDouble;
    public static ConfigEntry<Long> someLong;
    public static ConfigEntry<Boolean> someBool;
    public static ConfigEntry<String> someString;
    public static ListConfigEntry someDoubleList; // ListConfigEntries will change to use the params
    public static ListConfigEntry someStringList; // as ConfigEnties (ListConfigEntry -> ListConfigEntry<T>)

    public static void register() {
        MConfigBuilder builder = new MConfigBuilder(YourMod.MODID, "suffix-or-name", FileType.JSON5, false);

        someInt = builder.comment("Integer entry comment")
            .define("someInt", 10, 0, 100);

        builder.openCat("DecimalNumbers");
        someFloat = builder.comment("Float entry comment")
            .define("someFloat", 1.0f, 0.1f, 5.0f);

        someDouble = builder.comment("Double entry comment")
            .define("someDouble", 2.5, 0.0, 20.0);

        someLong = builder.comment("Long entry comment")
            .define("someLong", 123456789L, Long.MIN_VALUE, Long.MAX_VALUE);
        builder.closeCat();

        someBool = builder.comment("Boolean entry comment")
            .define("someBool", true);

        builder.comment("A category comment").openCat("Strings");
        someString = builder.comment("String entry comment")
            .define("someString", "Awawawa");

        someStringList = builder.comment("List entry comment - A string list")
            .defineList("someStringList", List.of("minecraft:grass_block", "minecraft:dirt", "minecraft:stone"), String.class);
        builder.closeCat();
        
        someDoubleList = builder.comment("List entry comment - A double list")
            .defineList("someDoubleList", List.of(4.2, 6.9, 6.7), Double.class);

        builder.build(); // Will create, load and update the config file depending on the situation
    }
}

TL;DR:

• Config creation logic is similar to SuperMartijn642's Config Lib, but isn't based on Suppliers making it editable through your own code instead of it being read-only
• MConfigBuilder takes:
  • Mod ID,
  • A file suffix/file name when subdirectory is made,
  • Config file type,
  • Boolean true/false do you want your config to be in a subfolder
  • Config Entries can be Integers, Floats, Doubles, Longs, Booleans, Strings or Lists of those same
  • value = builder.define(key, ...) defines a config key
  • value = builder.defineList(key, ...) defines a config list key
  • builder.openCat("CategoryName") opens a new category
    • Some config file types don't support categories (.properties)
  • builder.closeCat() closes last opened category
  • Categories can be put within eachother
  • .comment("example") can be used standalone with just builder.comment("An example");, or combined with entries or categories, e.g. value = builder.comment("A value").define(key, ...)
    • Some config file types don't support comments (.json)

package net.yourmod.mod;

import net.fabricmc.api.ModInitializer;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class YourMod implements ModInitializer {
    @Override
    public void onInitialize() {
        ExampleConfig.register(); // That's literally it
    }
}

Idk how to explain all that so good luck