Save data with acces for all player

A good approach to this would be usage of a Database, a common option being MySQL or SQLite. You would store your player data on a database and then write queries to fetch data from it, because all your player data is stored on a server and not for example, a clients computer, you can access it at your own discretion.

Should you use this option take into account security vulnerabilities may occur with poor implementation i.e. using connection details to the database on the client code itself which could easily be reversed and in turn give someone access to your database, I believe it's common practise to make a PHP backend (PHP requests to the server from your C# client code) for your game to run your queries as a more secure medium.