how to configure the source csv file or uri ?

The documentation should make it explicit that it is a 2 steps thing:

create a feed type on /admin/structure/feeds

create a feed on /admin/content/feed , and this is when the source is indicated.

Yes, the UX could be improved on this part a bit. Perhaps on the feed type pages there should be link somewhere to go to the feed listing (/admin/content/feed).

However, the steps are documented on https://www.drupal.org/docs/contributed-modules/feeds/creating-and-editi... →
Particular in the part "Overview" it is documented that you create a feed type first, then mapping and then create a feed.

Perhaps the wording could be improved a bit. Maybe the part of what's different from Drupal 7 is not so relevant. And I think it could help if there are headings for each step.

Something like this:

1. Create a feed type
   ...
2. Configure mapping
   ...
3. Add a feed
   ...