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 and later will 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, iPadOS will honour the Developer-intended Portrait mode. iPadOS will not override the App coding.