I have a SharePoint site with multiple lists, some of which have the same fields – a choice of products or countries.
How can I build the lists in a way that I configure the choice field once and use it in multiple lists, so that in the future, if I add a value to the choice, I add it only once?
If you go to Site Settings, under Galleries there is an option for Site Columns. You can create your choice list there. Then, under the Library Settings there is an option to Add From Existing Site Columns. You should be able to see and select your newly created column there.