Stage Plans in PRINCE2 are essential tools for the Project Manager, assisting in the day-to-day management and control of the project. Each Stage Plan provides the detail needed for each management stage of the project, including what work will be done, when it will be delivered, and who will be involved, thus facilitating effective project governance on a stage-by-stage basis. They do not necessarily match the duration of the Project Plan or get produced simultaneously with the Project Initiation Documentation.
PRINCE2 7 Foundation handbook and other official documents.

According to PRINCE2, a project is not considered part of 'business as usual'. The defining attributes of a project include it being a temporary organization that is created for a specific purpose, which contrasts with
'business as usual' that involves ongoing, routine activities within an organization. References: PRINCE2 Foundation manual, PRINCE2 official resources

The Plans theme in PRINCE2 is responsible for estimating the cost of delivering the project. This theme involves the development of plans that outline how, when, and by whom the project's objectives are to be achieved, including the necessary resources and associated costs. References: PRINCE2 Foundation official documentation and study materials.

Quality control events, such as testing, should be documented in the 'Quality Management Approach' according to PRINCE2 guidelines. This document outlines how quality will be managed throughout the project, detailing specific methods and events, including schedules for quality assurance activities and testing, to ensure that products meet their defined quality criteria.
= PRINCE2 7 Foundation Training Manual and official PRINCE2 website documentation

The Project Manager role in PRINCE2 is primarily responsible for ensuring that the project produces the required products, to the defined standards of quality and within the specified constraints of time and cost.
This central responsibility involves coordinating all project activities and resources to achieve the project objectives.
PRINCE2 7 Foundation resources and materials.