The whole app is downloaded to the iPhone. If the connection drops or there is not enough space, the installation stops.
What to do
Connect to Wi-Fi. Large apps (games) often fail to download over mobile data. It is best to turn off VPN for a while.
Free up space. You need roughly twice the app size. Check: Settings → General → iPhone Storage.
Delete the stuck icon and retry. Long-press the icon → Remove App → tap Install again.
Still not working?
If this app was already installed from the App Store, remove the old version first (banking and social apps keep your data on their servers). Still not working — message us.