You can find tasks using the search function. In the web app and desktop apps, the search bar is located in the navigation panel, above your smart lists. In the mobile app, the search bar is above the smart lists on your home screen.

The search results are divided into lists, folders, and tasks. 

To search archived lists and tasks, open their settings and select "Archive". From here you can use keywords to search your archived lists and tasks.

Before searching, choose whether you want to search within archived lists or tasks.