How do you install this app on a Mac using Google Chrome browser? It just says "Launch App"
Then you already have it installed. The app is a webpage that runs on your computer that is allowed to access your ledger device through the USB port. You are not visiting a webpage hosted by someone else, but instead visiting an internal webpage you have downloaded. It is however able to be updated by the ledger team, but someone would have to compromise their chrome web store key to be able to do that. And even if they did, an attacker would not be able to steal your balance because you have to manually approve your transactions on your device.