how do I rearrange icons in dock in macos big sur

Same way it has always worked. What may be different is the Force Touch Trackpad.

If you are not used to using it, the Magic Trackpad has a Force Touch feature that is another gesture. If you press too hard when you are trying to drag the icons, you may not be moving them, but invoking the Force Touch Gesture.

How to use the Force Touch trackpad - Apple Support

You might need to enable dragging in the Accessibility System Preferences, Pointer Control, Mouse & Trackpad tab, Trackpad Options button. I use it without drag lock so I don't have to click again to release.

I don't understand why they buried that gesture in the Accessibility preferences. Seems like basic functionality for a Trackpad. I can't imagine working without it enabled.