More information: https://www.macobserver.com/tips/how-...
The two ways to access widgets in macOS Big Sur are:
Click on the time and date in the top right-hand corner.
Two-finger swipe-left on your trackpad.
They will then show in the notification center on the right-hand side of your display.
How to Edit
Open widgets as described above.
Click 'Edit Widgets'
Click the '-' sign next to items you no longer want to be available in the Notification Center.
Press '+' sign next to any items you want to add. This can be moved by holding and dragging.
When you're happy with the arrangement, click 'Done'