How do Screen Time app limits work for multiple apps in Family Sharing?

Hi, I'm trying to work out how app limits on screen time work for my childrens' devices.

I currently have it set up as 'All Apps and Categories' for 3hrs - under the assumption that they will be able to use all the apps they have installed for a total time of three hours during the day - even if they have two or more apps open simultaneously. I am wondering if this assumption is correct OR, if instead, the way it works is that you can use one app for three hours or three apps open simultaneously for one hour?

I have been dealing with apple support on a related issue for several months and there is some confusion.

[Re-Titled by Moderator]

Original Title: Screen Time App Limits - additive?