Basic questions about Users, Permissions and the "User Mapping Dialog"

Hi,

I find myself a bit baffled by User and Permission model. The scenario is:

- In pgAdmin4, I have a server called "default".
- Under "default->Databases", I have my application database called
"foo", with a schema "public".
- Under "default->Login/Group Roles", I have amongst some other stuff,
the default user "postgres" and my application-specific user "app_user".
Naturally, app_user has access to the tables in foo.public.
- I login to pgAdmin4 as "abc(at)abc(dot)com".

Normally, when "abc(at)abc(dot)com" logs in, she is only interested in the
administrative aspects of "foo.public", such as looking at what sessions
are active and so on. That works fine as expected.

In exceptional circumstances, I would like for "abc(at)abc(dot)com" to be able to
use pgAdmin4 to look at (or even edit) the data in the tables as if she
were app_user. However, when I drill down to
"foo.public->Tables->sometable->View/Edit data", I get a permission denied
error. I guess this makes sense because there is no relationship between
abc(at)abc(dot)com (a pgAdmin4 user) and app_user (a Postgres user).

I've perused the pgAdmin4 docs and see that there is a section on the "User
Mapping Dialog", but I see no such dialog in the GUI.

Q1. Is that dialog the right place to give abc(at)abc(dot)com the ability to look
at the data which belongs to app_user?
Q2. If so, how do I make the dialog show up. Or am I barking up the wrong
tree?

Of course I have also poked around the User Management dialog and its docs,
to no avail, so a nudge in the right direction would be appreciated.

Thanks, Shaheed