Here goes a list of helpful resources that will make designing apps a lot easier. All of the resources are good to use for personal and commercial use.

Icons

https://material.io/resources/icons/?style=baseline

Material

https://material.io/archive/guidelines/resources/sticker-sheets-icons.html#sticker-sheets-icons-components

Apple

https://developer.apple.com/design/resources/

Microsoft

https://docs.microsoft.com/en-us/office/dev/add-ins/design/design-toolkits

Android

https://materialdesignkit.com/android-gui/

Design Resoures

https://ui8.net/

Illustrations

https://undraw.co/search

Images

https://unsplash.com/

Fonts

https://fonts.adobe.com/ https://fonts.google.com/

Other

https://xdresources.co/