Dynamically load json data to a game: What are the best practices?

Use Newtonsoft.Json it is the industry standard for C#/.NET JSON deserialisation/ serialisation.

Just read the file as a string using any of the normal .NET methods and use the Newtonsoft deserialisation method to map it onto an equivalent c# models i.e. POCOs.

https://www.newtonsoft.com/json/help/html/deserializeobject.htm

I suggest doing it by hand but if you are feeling lazy there are plenty of tools for generating the C# types which might get you most of the way there if there is a lot of data/ type variation. https://quicktype.io/csharp

Regarding the "remote storage", thats what databases are for, front a database with an api that provides an interface for the basic CRUD for the data you are trying to store. If SQL: You could use an ORM like Entity Frameworks or PetaPOCO to make talking to the database a little easier

https://docs.microsoft.com/en-us/ef/

Any SQL variant will do, I favour Postgres personally if I have a choice, otherwise MSSQL or MySQL will do the job fine.

https://www.postgresql.org/ https://www.microsoft.com/en-gb/sql-server/sql-server-2019 https://www.mysql.com/

Other than that you have NoSQL options like Mongo that may be useful. It really depends on your requirements. No real need for an ORM here they are pretty simple to set up CRUD the complexities really start to emerge when you are trying to work with a lot of data and how best to structure it as its a little more open ended than SQL.

The api can be written in what ever language and framework you want but C# .net/ web apis are pretty good and it would save you a lot of effort as you could share the DTOs in a single library across your two projects meaning less maintenance burden.