Working with data on Roblox is very annoying and painful, but in my experience it is usually wise to use a DataStore module, specifically I use ProfileService.
You can read all about it on the provided link, has a ton of great perks and benefits and is easy to use with plenty of documentation.
As for how to code the system, the global bank will need to have a key such as “BANK_001” and each player will need to have their own individual key. Whenever a player makes a purchase you just need to add the value into your bank’s saved key.
Here are some youtube videos on ProfileService which you may find usefull: ProfileService - YouTube