A field widget stores some text, a slider widget stores a number, a canvas widget stores a picture, that’s about all there is to it.
If you see a canvas widget’s content changing between different frames of animation, then either the canvas is being redrawn by code (as in the Canvases card in the built-in tutorial deck), or there’s some other card or canvas somewhere that has all the different frames, which are being copied into the canvas to create animation.
Decker comes with an example deck called Puppeteer, which includes a module you can include in your own decks to do this kind of animation.