I want to make a link between two spreadsheets, like if I click on a code in a cell, it opens the spreadsheet which the code refers/links to.
You want to enter a link in a Google spreadsheet.
The syntax of the command is:
The key is that the Google spreadsheet you want to open has a URL.
I use the Google Sheet Add-on called Sheetgo. It's easy to use and after installing, gives you the ability to link sheet in many way, even if you need to connect multiple sheets.