Multisite config directories of subsides
It's a little bit up to you how you manage your config. In my multisites I have a ../config directory outside the web root. Each site instance then has it's own folder in there. And inside each one I split the config per environment.

shared/ holds the config that is similar on all environments. local/ holds all the extra config for local development. And live/ holds the extra config for the live site. This can be achieved using the Config Split module.

project/
├── config/
│   ├── bar/
│   │   ├── live/
│   │   ├── local/           
│   │   └── shared/
│   └── foo/
│       ├── live/
│       ├── local/
│       └── shared/
└── web/
    └── sites/
        ├── bar/
        └── foo/


The settings.php config for the foo/ instance then looks like this:

$config_directories['sync'] = '../config/foo/shared';


And the Git-ignored settings.local.php on my local machine looks like this:

$config['config_split.config_split.local']['status'] = TRUE;
$config['config_split.config_split.live']['status'] = FALSE;


Whereas on the live server it looks like this:

$config['config_split.config_split.local']['status'] = FALSE;
$config['config_split.config_split.live']['status'] = TRUE;