Hi,
we can't make an object of an interface, but we can create an object of a class that implements this interface.
You can find an explanation how DriverManager.getConnection works in the JDBC specyfication:
http://jcp.org/aboutJava/communityprocess/final/jsr221/index.html
getConnection — the method the JDBC client invokes to establish a connection.
The invocation includes a JDBC URL, which the DriverManager passes to each
driver in its list until it finds one whose Driver.connect method recognizes the
URL. That driver returns a Connection object to the DriverManager, which in
turn passes it to the application.
DriverManager just passes the URL to each registered driver, if one driver accepts the url, then this driver creates a connection object
(that implements Connection interface) and returns this object to the driver manager.