The Site Column helps you by providing a reusable column within your site and sub sites. Once you create a site column at our top level site , we can use that site column in the sub sites as well.
Go to the Site Settings
Next Under ” Galleries” , click “Site columns”
On this page that is displayed, a list with all existing site columns is displayed. Click “Create” button to create a new site column.
If you Scrolle down can see created site column
Now create custom list( previous post how we created custom list in our sharepoint site), and then Go to the list Settings.
Click on Add from existing site column
Select Group name then you can find our site column(FirstName) and add and Click ok.
We can see our site column added into our list