Hi,
There are two things you need to understand and differentiate.
1. Creating a copy of company:
If you want to test your setup and data by posting some transactions you can achieve it by creating a copy of your current company. By creating a copy of company, all your current company's data will be copied to a new company, master, setup and transactions.
To create a copy of company search companies page. Then on the companies list page select the company that you want to copy and then use action Copy company. Name the new company and wait for few minutes. After a company is created you can switch to that company by going to Settings icon at the top right and then in the company change to new company. Page will be refreshed and you will be in other company.
2. Creating a sandbox from Production:
By creating a copy of sandbox from production, all of your companies along with the extensions that are installed on production will be copied to a new environment called sandbox. You can also do the testing in sandbox. But primarily sandbox is more used for development testing. Because if you install an extension on production it will effect functionality for all companies including live company. So to test an extension we create a new sandbox either a copy of production or a new depending upon our requirement and test extension.
In short for your need you can choose option 1.
As you have written if you have created a copy of production then in sandbox you should be able to see the live company. Just try to change the company and you can see it.
