How to connect to and start a specific database using dgmgrl

The data guard broker allows you to connect to any of the participating members of the configuration:

oracle@myserver:[hdal]# dgmgrl / as sysdba
DGMGRL for Linux: Release 12.2.0.1.0 - Production on Fri Jul 31 10:44:47 2020

Copyright (c) 1982, 2017, Oracle and/or its affiliates.  All rights reserved.

Welcome to DGMGRL, type "help" for information.
Connected to "hdal"
Connected as SYSDBA.
DGMGRL> show configuration

Configuration - DGConfig1

  Protection Mode: MaxPerformance
  Members:
  hdal      - Primary database
    hdal_stb  - Physical standby database
      hdal_tstb - Physical standby database (receiving current redo)

Fast-Start Failover: DISABLED

Configuration Status:
SUCCESS   (status updated 51 seconds ago)

DGMGRL> connect sys@hdal_stb as sysdba
Password:
Connected to "hdal_stb"
Connected as SYSDBA.

Note that the password files must be identical for all members of the configuration.
You can startup and shutdown individual members of your configuration without leaving the data guard broker interface, for example:

DGMGRL> startup mount
ORACLE instance started.
Database mounted.

Documentation on how to use Oracle 12.2 version of dgmgrl is found here