Can I ensure an installed application only runs when I open it? That is, I want to ensure it consumes no system resources (data, RAM and CPU) unless I open it.

Intuitively, if I block all notifications from an app, this might happen. But then again, it might not.

Does it? If not, is there a way to achieve that?
So you want to ensure that certain apps do not start themselves automatically. There are several apps available to control auto-start behavior. The currently best ranked app is Brevent – available in the Playstore as well as in my F-Droid repo. Other than most other apps, this app does not require root to do its job.

Ensuring the app doesn't start unless you tell it to also makes sure it does not consume any resources (except for space on storage) unless you explicitly run it.