How do I stack large widgets iOS 14?

Use both fingers: Hold the large widget with one finger, and use another finger to swipe it across the screen. Then position it atop some other widget to create a stack.

How do I add widgets to stack?

Long press on a blank area of the ‌Home Screen‌ or any additional page of apps. Once in jiggle mode, tap the plus (+) button in the top-left corner of the screen. Scroll through the ‌widgets‌ card and press a widget that you want to include in your widget stack, then drag it onto the ‌Home Screen‌.