How client - backend/server conversation should work?

Hello,

i am working on a multiplayer rpg game and i have some problems with understanding client-server conversation principies.

For example in a rpg game a player has inventory (List of Items), some are equiped, some are in the bag. On scene load i load the inventory from my database and set equiped items to player equip slots. Next i want to equip a item from the bag. I do a request do database, there some process will happen (Has player item that should be equiped? If yes, check a item...

How client - backend/server conversation should work?