How do you view portrait apps

Apps may, or may not, support both Portrait & Landscape modes - this being at the discretion of the App developer. Some Apps may only support one or other screen mode - and will always launch in this mode if the other is not supported.

Not all App developers choose to support Landscape mode in Apps intended for an iPhone. Some Apps that are designed for iPhone may only work in Portrait orientation - and some will not re-scale to the larger screen of iPad. In either case, there is nothing that you can do to change the manner in which the App displays on your iPad.

You need to direct your feedback - and feature request for full iPad support - to the individual App Developer(s):

How to contact an app developer - Apple Support

I hope this is helpful in answering your question.

Apple has announced that iPadOS 15, presumably to be released this fall, will allow the user to view these apps (that are stuck in portrait mode) in a narrow vertical strip in the center of the iPad when in landscape orientation.