Make sure you've updated your iPhone to iOS 14. Hold down your finger anywhere on your screen (or on an app and select "Edit Home Screen") until the apps wiggle.
Tap the + icon in the top left corner.
Select the Apple or third-party widget you'd like, pick a size (small square, medium horizontal rectangle, and large square), and tap Add Widget to add it to your home screen.
Move your widget to the location you want on the home screen and then select Done in the top right corner to set it in place.