Correct way to add fields

yes. It confuses me, too.

I have to create a content type OUTSIDE of groups, then assign it as plugin.
(I have not found a way to create a content type INSIDE of a group.)

But it is different with fields: I can create them outside and inside.
But only if I create fields inside i can reuse these fields?! Even though I have already added the entire content type?!

What is the logic/purpose of this?
What are the implications?

So either both can be created inside and outside or just outside.
But please let me reuse fields from an added content type - even if they were created outside.