I see that I an disable auto wordpress core updates on a single site page, but is it possible to do this with multiple sites selected anywhere?
You can control this setting on a Dashboard level on the Settings > General Settings page.
In case you were asking about disabling WP Core updates that may occur directly on the child site - you can do that by utilizing the MainWP Code Snippets Extension.
Simply add this line as shown in the screenshot, select Save to wp-config.php option, and click Save & Execute Snippet:
define( 'WP_AUTO_UPDATE_CORE', false);
More information about the Code Snippet extension: