Mindy,
I will divide your question into two parts :
1. Viewing Returning / New users
2. Adding new metrics in reports.
1. Viewing Returning / New users
You can use the New / Established comparison to view the split on the same screen.
https://photos.app.goo.gl/KZxSgdx9SkgnQeyk6
By using comparison, you can view all existing reports data split by new / returning.
(Type established in the search bar to quickly access the comparison )
https://photos.app.goo.gl/ggZvQmMskyD34dGA6
2. Adding new metrics in reports.
To add new metrics to the report. You can “Create new cards”. ( This is different from “Adding a card).
There are two types of reports in GA4.
Overview Report: You can only Add Cards.
Detailed Report: You can create new Cards.
In your screenshot, you are looking at the Overview Report, You can add existing cards in that by clicking on the Edit icon on the right and then clicking on Add Cards on the panel on the right.
https://photos.app.goo.gl/HtBK5ugiNDWD8RGE9
If you want to Create new cards with some metrics. You can customize any Detailed Report. On clicking the customize icon on right, you will see the Create New Card option
https://photos.app.goo.gl/qzkmzcBZQg2VQyr89
Clicking on Create new Card will give you screen for customizing card .
https://photos.app.goo.gl/V1UU5ZodGhQuJjX18
Here you can add Dimensions / metrics and filters to look at the metrics you are after.
Let me know if that solves your challenge