The database paths are normally defined in the create database command when specifying automatic storage locations for each database. If not specified, it defaults to the default database path, which is defined in the DBM (instance) config (which is usually the home directory of the instance owner). For Windows, it is a little different, as you will see for the SAMPLE database that gets automatically installed on Windows.
The file extensions depend on what files you are talking about. For DMS tablespace files (containers) there are no fixed extensions if you specify them yourself (not using automatic storage). Not sure why you need to know the extensions (unlike Oracle, it is best to keep your hands off the data, index, etc files and only interface with DB2 utilities).