Site with multiple databases

Opencart 2, selling books. The task is to load the database on one domain by the decision (desire) of the client that he will determine. I mean. If the client wants to buy one base in Moscow, if in other countries. It's different. I suppose you need to implement a drop-down menu with the choice of city in the template, as well as a pop-up menu when loading a site where you offer a choice: “Where do you want to buy a product?” and the choice of cities (countries).
And also to be able to further supplement these cities and remove them by connecting duplicates of the same base. The goods are identical, but the problem is that not all branches will have the same availability of goods, different shipping costs and different people will oversee projects.
So that it all works on smartphones...
There may be another way to do this, so suggest... Goals are simple and not expensive.