What do we have to prepare before upload in Amazon

Create these your application icon for various dimension in png format:

1. Small icon 114 x 114px PNG (with transparency)
2. Large icon 512 x 512px PNG (with transparency)

Promotional image (Recommended) 1024 x 500px (landscape only) PNG or JPG
Create 3 Screen shoot 800X480 or 480x800.

What do we have to prepare before upload our application in Google Play Store

Create your application icon in 512x512 px. in Android Studio, use Image Asset to generate application icon (include for Google Play Store) 

Google play publishing a new application, from dashboarrd
1. Set privacy policy (url, privacy policy must image)
2. App access
3. Ads
4. Content rating
5. Target audience
6. News apps
7. Data safety
8. Government apps
9. Financial features
10. Health
11. Select an app category and provide contact details
12. Set up your store listing (Store presence -> Main store lisiting)
13. Select countries and regions
14. Create closed & Open testing release (the same version)
15. Create a new release
16. Preview and confirm the release
17. Send the release to Google for review

Store presence -> Main store lisiting
1. App name
2. Short description
3. Full description
4. App icon
5. Feature graphic demension 1024 w x 500 h in JPG or 24-bit PNG (no alpha
6. Phone screenshots (8 images)

https://developer.android.com/distribute/console