Hi, nice to be here, I have some ideas for you.
You can do two types of economy, to create a shop, the virtual one or the physical one
The virtual one is the "money" that is shown to you on the scoreboard
Or the physical ones that you can use objects like coins, for example emeralds, flowers or other
For a physical shop you can use a plugin, for example "shopkeeper" which allows you to exchange coins with a personalized name if you want, with personalized objects. I recommend the "coins" plugin that allows you to manage both physical and virtual money with a custom name
For a virtual shop you can use a plugin, for example "economy shop gui" that allows you to have exchanges through virtual coins, such as those of the essentials
For the keys you can use the free "exellent crates" plugin
For mob drops you can search for a plugin on the internet, it seems there are some
