How can i save the games inventory?

In general, .saving and loading data in Unity is no different than in other C# applications. You'll need:

• A serializable class to hold the data you want to save -- anything [System.Serializable]

• A serializer/deserializer, such as .NET's System.Runtime.Serialization.Formatters.Binary.BinaryFormatter

• A stream to (de)serialize -- most likely a FileStream

  ---

I'll provide a simple, but complete, example that demonstrates saving and loading a custom inventory class below. First, a preview:

To test, you'll need to add the Player component to a GameObject in your scene, then enter play mode. Pressing 1 adds to the inventory, 2 removes a random item, 3 prints the inventory to the console, 4 saves the current inventory, and 5 loads the saved inventory.

An inventory is just a list of items -- this is what we'll be serializing as our save data.

Assets/Inventory.cs

 using System.Collections.Generic;
 [System.Serializable]
 public class Inventory
 {
     public List<Item> items;
     public Inventory() { items = new List<Item>(); }
 }

An item has a few simple properties. Note that it also has the Serializable attribute.

Assets/Item.cs

[System.Serializable] public class Item { public string description; public int value;

 public Item(string description_, int value_)
 {
     description = description_;
     value = value_;
 }

 public override string ToString() { return $"{description} -- {value}"; }

}

The player class holds the save/load logic, and some code to help test. The SaveInventory and LoadInventory functions are what's most important.

Assets/Player.cs

 using System.IO;
 using UnityEngine;
 
 public class Player : MonoBehaviour
 {
     public Inventory inventory = new Inventory();
     string InventorySavePath => Application.persistentDataPath + "/inventory.save";
 
     void Update()
     {
         if (Input.GetKeyDown(KeyCode.Alpha1)) { AddItemToInventory(); }
         if (Input.GetKeyDown(KeyCode.Alpha2))
         {
             if (inventory.items.Count > 0) { RemoveItemFromInventory(); }
         }
         if (Input.GetKeyDown(KeyCode.Alpha3)) { DebugInventory(); }
         if (Input.GetKeyDown(KeyCode.Alpha4)) { SaveInventory(); }
         if (Input.GetKeyDown(KeyCode.Alpha5)) { LoadInventory(); }
     }
 
     void DebugInventory()
     {
         foreach (var item in inventory.items)
         {
             Debug.Log(item);
         }
     }
 
     void AddItemToInventory()
     {
         var item = new Item(System.Guid.NewGuid().ToString(), Random.Range(1, 100));
         Debug.Log($"Adding item: {item}");
         inventory.items.Add(item);
     }
 
     void RemoveItemFromInventory()
     {
         var randomIdx = Random.Range(0, inventory.items.Count);
         var item = inventory.items[randomIdx];
         Debug.Log($"Removing item: {item}");
         inventory.items.RemoveAt(randomIdx);
     }
 
     void SaveInventory()
     {
         Debug.Log($"Saving inventory to file @ {InventorySavePath}");
         var stream = new FileStream(InventorySavePath, FileMode.Create, FileAccess.Write);
         var formatter = new System.Runtime.Serialization.Formatters.Binary.BinaryFormatter();
         formatter.Serialize(stream, inventory);
         stream.Close();
     }
 
     void LoadInventory()
     {
         Debug.Log($"Loading inventory from file @ {InventorySavePath}");
         if (File.Exists(InventorySavePath))
         {
 
             var stream = new FileStream(InventorySavePath, FileMode.Open, FileAccess.Read);
             var formatter = new System.Runtime.Serialization.Formatters.Binary.BinaryFormatter();
             inventory = (Inventory)formatter.Deserialize(stream);
             stream.Close();
         }
         else { Debug.LogError("Save file does not exist!"); }
     }
 }

Hope this helps!