You are not the first - and neither will you be the last - to complain of this issue. The problem that you describe is not an issue with your iPad and iPadOS, but is instead an App coding issue that can only be resolved by the respective App Developer.
If screen rotation proves to be problem with some Apps - and not others - you need to be aware that not all Apps are coded 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 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. For Apps that the Developer intends to operate in Portrait screen orientation, your only available course of action is to contact the App Developer - and request that the App is updated to be useable in Landscape mode.
How to contact an app developer - Apple Support