What processes (subcontracting), what products and services are relevant to the operation of your organization and are acquired from an external provider? Relevant in the sense: They help to make a difference; or avoid flaws that the customer recognizes and negatively values. In other words, you do not need to apply the requirements of the standard to everything you buy.
So 8.4.1 is about qualifying a potential supplier, is about selecting a supplier to satisfy a particular need:
And periodically evaluate supplier performance based on delivery results.
About 8.4.2 it better specifies the part of the control mentioned above: what to control during delivery, how to evaluate, who does what, what specifications, what records are used.
8.4.3 is about the information that goes on orders or contracts: The description of the product, service or process (1) Methods, processes or equipment to be used or to be followed in the provision of services (2) Relevant quality control criteria (3) In cases where there is a requirement of competence, mention it (c) It can sometimes make sense to describe means of contact, contact persons, authorities (d) It may make sense to define quality control rules during the provision of the service (e). It is to clarify that the company can go to the supplier's premises to carry out an audit or a quality control (f)
The following material will provide you more information: