If screen rotation proves to be problem with some Apps - and not others - you need to be aware that not all Apps are coded by the respective Developer to operate in both Portrait and Landscape screen modes. Apps that have a fixed screen orientation will always appear in the orientation determined by the App Developer.
For iPhone Apps that are compatible with iPad, iPadOS15 will automatically rotate the App to display in Landscape screen orientation. There is no manual configuration or setting to enable this feature. However, for 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.
Clearly Apps that force Portrait orientation can be problematic when using an iPad with a Magic or folio keyboard - that, by design, requires the iPad to be used in Landscape.
Regrettably, some App Developers appear blind to this issue; if you are affected by this problem, you would be best advised to contact the App Developer - and request that the App is updated to support Landscape.
How to contact an app developer - Apple Support