How to list all foreign servers?

You can query the system catalog pg_foreign_server, e.g.:

select 
    srvname as name, 
    srvowner::regrole as owner, 
    fdwname as wrapper, 
    srvoptions as options
from pg_foreign_server
join pg_foreign_data_wrapper w on w.oid = srvfdw;

      name      |  owner   |   wrapper    |                options                 
----------------+----------+--------------+----------------------------------------
 csv_server     | postgres | file_fdw     | 
 foreign_server | postgres | postgres_fdw | {host=localhost,port=5432,dbname=test}
(2 rows)