Not sure about DuckDuckGo, but for Google you just search something (only desktop version has the option when I do it on mobile), then click the three little dots next to whatever URL you want in the results. It'll pop up a little "more options" window. From there you have to click the little down arrow in the top right of that window and it will reveal a "cached" button to click. There might be an easier way (and it used to not be as "hidden" as it is now beneath the menus) but this is how I know to do it.