You can work out a number of things by looking at the various out-of-the-box URLs in SharePoint. For example, to work out the ID of a content type, go to the content type gallery, hover over a content type, and observe the URL. The content type ID is the last query string parameter as shown below.
One way to work out the site column ID is to go to the setting of a content type where the site column is being used. Hover over the site column and the ID is shown in the URL as below.
Going to the site column gallery won’t work as that will just give you the internal name of the site column/field.