How do you check for apps running in the background

As the title says how do you see which apps are running, even stuff in the background?

On Android, the easiest way to see what’s actively running in the background is to open Settings, go to Developer options, then Running services. It shows live RAM usage and lets you force-stop apps from there.

Not sure how to do it on an iphone, I’ve never had one

What are we talking about? PC? MAC? Mobile?