now they've invented a special "format" that only can only be installed through their app

If I'm not mistaken, that's the .apkm bundles right? That's not their own format but another one Google invented to be able to dynamically install only the relevant parts of an app for a specific device type, locale, etc.

Android can't install this on it's own (I wonder who's fault that is 🙄) but it can be installed via adb or through special installer apps like the Play Store, Aurora or I'm assuming the APKMirror app