Shane Young did a great video on why you should ditch groups and use containers in Power Apps Canvas apps.
Video of Shane Young:
This is 100% true, but we need to dive deeper into that topic:
Use HORIZONTAL/VERTICAL #CONTAINERS in EVERY App. They are not just there for responsive scenarios, but make your life sooo much easier in every kind of app (but yes, they are especially awesome for responsive apps).
In a quick timelapse I set up the main layout without going into detail. Afterwards we go a little more into detail and build a PERFECTLY centered and repsonsive (this is more of a byproduct) popup with just 2 containers.