Do I have to empty my trash and Junk/Spam folders?

No, it's automated.

By default, the Junk/Caught-Spam folder, and the Trash folder are 'protected' official folders that are automatically trimmed/emptied. The Trash folder is emptied daily whereas the Junk/Caught-Spam folder is kept trimmed to a 7-day history (non-adjustable).

All other folders (such as those you create and the INBOX) are not content-managed with regards to age trimming and emptying.

Pro-Tip: If you notice your 'Trash' folder or 'Junk' or 'Spam' folder is not being automatically emptied on your PC/MAC, then your folder is a "local" folder and not actually one of the system-reserved folders described above.