100% of what Pizza Pie says above is spot-on.
ALSO… use ScriptableObjects to define the things that you can possess in your inventory, but keep that completely separate for the bookkeeping of what you actually own.
I recommend just using serializable classes and JSON, since it writes a text output and is SUPER-easy to debug and find your errors in, as well as to hand-hack test stuff into.
Also, there are many problems with Unity “tiny lite” built-in JSON:
I highly suggest staying away from Unity’s JSON “tiny lite” package. It’s really not very capable at all and will silently fail on very common data structures, such as Dictionaries and Hashes and ALL properties.
Instead grab Newtonsoft JSON .NET off the asset store for free, or else install it from the Unity Package Manager (Window → Package Manager).
PS: for folks howling about how it will “add too much size” to your game, JSON .NET is like 307k in size, and it has the important advantage that it actually works the way you expect a JSON serializer to work in the year 2021.
And here is my general Load/Save steps:
Don’t use the binary formatter/serializer: it is insecure, it cannot be made secure, and it makes debugging very difficult, plus it actually will NOT prevent people from modifying your save data on their computers.
When loading, you can never re-create a MonoBehaviour or ScriptableObject instance directly from JSON. The reason is they are hybrid C# and native engine objects, and when the JSON package calls new to make one, it cannot make the native engine portion of the object.
Instead you must first create the MonoBehaviour using AddComponent() on a GameObject instance, or use ScriptableObject.CreateInstance() to make your SO, then use the appropriate JSON “populate object” call to fill in its public fields.