How to download apps that are over 200 mb using my mobile data?

To use data to download apps (iOSs 13 or higher), turn off automatic app downloading:

Update apps manually --> How to manually update apps on your Apple device - Apple Support

Settings > App Store (formerly iTunes & App Store) > App Downloads

In the cellular data section > select an option:

Always Allow

Ask If Over 200 MB

Always Ask

Basically my underlining that requirement was to intimate you can't do it on your device which can't go higher than iOS 12.

If you still have the iPhone 6, you won't be able to use mobile data for this unless something has changed very recently.