I was just wondering if app extension (f.e. stickers) affects containing app size in the App Store? As far as I can see in the build package, there are resources, main executable file and executable file for the extension. So it probably won't affect final app size, but I'm not sure and cannot find any information in the internet. Can anybody help me with this question?
Yes, the app extensions will affect the final ipa file size.
Build out your two .ipa files. One for the project without the app extension and one with the app extension. Compare those two file sizes to see the size difference.
To see the size of the .appex, rename your .ipa to .zip and double click the zip file to extract it. Open the extracted folder and control+click the .ios file. You can now look inside that folder and see what is changing.