iPad iOS 15 viewing iPhone apps in landscape mode

For compatible Apps that are intended for iPhone, iPadOS15 will now automatically rotate the App to display in Landscape screen orientation. There is no manual configuration or setting to enable this feature.

However, Apps designed for iPad that are expressly coded to force Portrait screen orientation, iPadOS15 will honour the intended Portrait mode. iPadOS15 will not override the App coding.

You don’t.

Many iPhone apps DO NOT have the ability to rotate their app/s to landscape orientation.

This feature has to be coded into the app by the third party app developer.

Many iPhone ONLY third party app developers choose NOT to implement the landscape orientation feature.

You will need to reach out to the applicable third party iPhone app developers and give them your feedback on this issue.

This is a very old and very long-standing issue.

Good Luck!