Storing data on a server to be used for player currency in a multiplayer game

short and concise question, where can I find out how to do this.

Depends on what you are looking for, You can go with AWS unity SDK and use DynamoDB(NoSQL) Database or Use Google Cloud platform and their DataStore (NoSql) although here you will need a server to interface with it datastore.
Or simply use traditional Mysql and just make HTTP get/put request to save and fetch data.
Again your question is too broad to have a straightforward answer.
You can also use google play SDK for unity, and save high score data using that (here you won’t have to worry about backend at all)