The systemd service mautrix-hangouts.service expects config and registration files here.

Quick Setup:
  ( adapted from https://github.com/tulir/mautrix-hangouts/wiki/Bridge-setup )

1. Copy example config and ensure service can read it
 sudo cp /usr/share/mautrix-hangouts/example-config.yaml /etc/synapse/mautrix-hangouts/config.yaml
 sudo chown mautrix-hangouts /etc/synapse/mautrix-hangouts/config.yaml

2. Update the config to your liking. You'll at least need to change the
  homeserver settings. You also need to add your homeserver in the permissions
  section and the auth server.

3. Generate the registration:
 sudo mautrix-hangouts -g -c /etc/synapse/mautrix-hangouts/config.yaml \
	  -r /etc/synapse/mautrix-hangouts/registration.yaml

4. Add the registration file to Synapse's homeserver.yaml:
 #homeserver.yaml
 app_service_config_files:
    - /etc/synapse/mautrix-hangouts/registration.yaml

5. Restart synapse
 sudo systemctl restart synapse

6. Create/update the database:
 sudo mautrix-hangouts-db-upgrade

7. Start the bridge:
 sudo systemctl start mautrix-hangouts