How to learn application address via database? and vice versa

When you are connected to a database, you can see the applications running on it.

"select host, domain from FND_NODES" and take port number from your tnsnames.ora. To see which nodes are active support_cp, support_forms, support_web, support_admin columns can be checked.

Here is your application address:
http://host.domain:"8000+(portnumber-1521)"

When you are connected to application and have system administrator responsibility you can see the database connection info;

system administrator>Oracle Applications Manager>Dashboard

here your tns info:

"nameyouwant"=
(DESCRIPTION=
(ADDRESS=
(PROTOCOL=TCP)
(HOST="host.domain")
(PORT="(x-8000)+1521")
)
(CONNECT_DATA=
(SID="Applications System:")
)
)

x: Application address no
Note: Remove the " signs