Use counts, not ownership
Article 21 speaks of the systems the entity uses for its activities or for its services [1]. A transport management system, warehouse system or telematics platform that you purchase therefore falls under it, even if it runs at the supplier.
What you do per system
Include the system in your inventory and risk analysis [2], and arrange access, backups, logging and the agreements with the supplier. The ministerial regulation for your sector also asks you to segment your systems into networks or zones on the basis of your risk analysis and, where appropriate, relative to systems of third parties [5], and to take measures against malicious software [6]. For transport and chemicals that is the Cyberbeveiligingsregeling IenW [3]; for postal and courier services and food, among others, regulations with the same wording apply [4].
What stays organisation-wide
Cyber hygiene and training, secure communication, management body approval, the reporting duty and registration are duties of the organisation as a whole, not per system.