The In-place Records Management feature must be activated by the site collection administrator.

When a document is declared a record, it becomes locked for use and cannot be deleted or modified.

Declaring a record manually

Select the document by checking the box to the left of the file.

From the ellipses (...) select More--Compliance Details

If the RM feature has been enabled and you have the permission to declare records you will see the option to "Declare as an in place record"

Select this and accept the pop-up. Your document is now a record

In the modern experience, records are marked with a circle and arrow

You can also add the 'Declared Record' column to your library to see when the document became a record.