Howdy!
So, a few things here: first this is a pretty straightforward React question of where to put in an Ajax call to another site and grab the data. And can help with that if it’s something you’re not familiar with.
The complication is I am not sure, but it is possible withSelect may work with multisite. I don’t think it does but that would be a question to verify in the Core Editor WP Slack (happy to ask on your behalf if you’re not on the WP Slack, but if you are a good question for there).
The one thing tho I’m not completely sure is are you trying to pull in content from across multiple sites or just content from the site where the block is used?