You're welcome.
Looking at the support page I posted (section "Use another app while on a call", point #2) it says "To return to the call, tap the green call indicator at the top of the screen".
You're right, this could be better explained in detail, as the situation varies depending on the iPhone model you are using.
In fact in the old models with the home button (i.e. 6s, 7, 8), all the status bar at the top turns green, while in the more recent models (model X or later) there is the notch, therefore only on the left (where the time is indicated) turns green.
Thank you for reporting this inaccuracy in the guide. I'll report this to Apple.
Regards
Giulio