dblink EXTENSION is a technique to connect another database and make operation of this database so to do that you need:

1-Create a dblink extention:

CREATE EXTENSION dblink;

2-Make your operation:

For exemple Select some attribute from another table in another database:

SELECT * FROM 
dblink ('dbname = bd_distance port = 5432 host = 10.6.6.6 user = username 
password = passw@rd', 'SELECT id, code FROM schema.table') 
AS newTable(id INTEGER, code character varying);

FDW is an implimentation of dblink it is more helpful, so to use it:

1-Create an extention:

CREATE EXTENSION postgres_fdw;

2-Create SERVER:

CREATE SERVER name_srv FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host 'hostname', 
dbname 'bd_name', port '5432');

3-Create user mapping for postgres server

CREATE USER MAPPING FOR postgres SERVER name_srv OPTIONS(user 'postgres', password 'password');

4-Create foreign table:

CREATE FOREIGN TABLE table_foreign (id INTEGER, code character varying) 
SERVER name_srv OPTIONS(schema_name 'schema', table_name 'table');

5-use this foreign table like it is in your database:

SELECT * FROM table_foreign;

To access complete schema of server db instead of single table. Follow below steps:

1. Create EXTENSION :

    CREATE EXTENSION postgres_fdw;

2. Create SERVER :

    CREATE SERVER server_name FOREIGN DATA WRAPPER postgres_fdw OPTIONS (host 'host_ip', 
    dbname 'db_name', port 'port_number');

3. Create USER MAPPING:

    CREATE USER MAPPING FOR CURRENT_USER
    SERVER server_name
    OPTIONS (user 'user_name', password 'password');

4. Create new schema to access schema of server DB:

   CREATE SCHEMA schema_name;

5. Import server schema:

     IMPORT FOREIGN SCHEMA schema_name_to_import_from_remote_db
     FROM SERVER server_name
     INTO schema_name;

6. Access any table of server schema:

    SELECT * FROM schema_name.table_name; 

This can be used to access multiple schema of remote DB.