There are some SharePoint technical names you need to know: "Site Content Type", "List Content Type", "Site column" and "List column".
Site and List Content Types
List Columns or Site Columns
The site content types and site columns are be reused in the site. When adding a site content type to a list/library, SharePoint copies the site content type locally into the list/library as a list content type. And, when adding a site column to the list/library, it copies the site column locally onto the list as a list column.
If you go to site settings -> site content types -> edit column in the site content type, or navigate to site settings -> sit columns -> edit column, you are editing a site column. To sync the changes to list columns based on this site column, click "Yes" on "Update all list columns based on this site column".
When changing it in Column default value settings at the library, you are editing a list column.
When editing the column from Column default value settings in the library or Library Settings > Columns > edit, you are editing the list column.
But using the Column default value settings, we can specify a default value for specific level.
For example, we have a column with default value "a" in the list and a folder1 in the list. You can define the default value of a column to "b" in the folder1. When creating items/folders in the list, the default value is "a". But when creating items/folders in the folder1, the default value will be changed to "b".
The Column default value settings will overwrite the default value of the column based on the defined level.
Information about Column default value settings