At the same time the only apps I cannot select & copy any text from are YouTube and GoogleMaps. The comments people leave there are so valuable for Google?

I just tried it, I seem to be able to copy text just fine from both apps.

Maybe it's because you tried iOS apps? It doesn't work in Android apps.

I misspoke. I meant I can copy & paste on the web. On iOS I can copy paste from Google maps, but not Youtube. Maybe there just isn't a good enough use case for it, that's why it's not supported everywhere?