How can I make a pop Up launched while starting an app?

You cannot do it in the launchscreen, but you could define a conditional first instructional screen (maybe with the same background image as launch screen).

You would propose user whether he/she wishes to see it next time and save the answer in user prefernecs.

It could be useful to allow user to retore the possibility in app's preferences.

See this 3 year old thread for details on how to do.

h ttps://forums.developer.apple.com/thread/23067

Are you sure you understand the difference between simply starting/running an app, and when an app is resumed/made active?

I'd expect the latter to occur as a routine, not the former.

As well, is there some reason the native launchscreen isn't enough?