Something to keep in mind is there a going to be multiple syncing services running on your phone, and they're independent from each other. Google's integral syncing will involve just Google services -- i.e. Gmail, Google Photos, Google Drive, Android settings/configurations, etc. If you have an extensive collection of audio files however, that's not necessarily going to auto-synced into your online Google account
unless you've previously uploaded your audio library up into Google Music (which will be soon have to be migrated into Youtube Music). So auto-syncing any data/files you're referring to are going to be a conditional matter. Another example is your text messages being auto-synced will be dependent on which text messaging service and app you're using -- i.e. WhatsApp is going to be syncing with WhatsApp online servers, not your Google account.
And that brings up another really important point, your phone is typically just on the receiving end of your various services. Your email/contacts/calendar are typically stored and maintained by online servers, with different apps on your phone just being a way to display the data. It's your online Google account that's the original source for your various Google services. You can set up different mobile devices to sync with your online Google account, that's how they sync with each other. The two different devices don't auto-sync with each other, they both sync with your online Google account. You can manually disable auto-syncing in your Settings menu, and then at that point the data on your phones exist only on your phones, (a very risky measure). Manually syncing one phone with another involves a lot of effort and discipline on your part.
Also, when it involves third-party apps (apps you install), their isn't going to be auto-synced into your Google account. So when it comes to a game, if it involves a constant online connection than your account with that particular game will 'probably' be tied to your online account. But a lot of game apps are local only so their respective data will exist only on the phone the app is installed on.
So when it comes to data on your phones, there's a lot of variability involved so it's actually a complicated issue. Some of it will be auto-synced if you have a common Google account set up on both phones; some of it will require a service, like Syncthing, to be installed and configured to share data between multiple sources; some of it isn't readily accessible (unless you root your device and then go through learning process to determine how to access the otherwise inaccessible data files/folders).
https://play.google.com/store/apps/details?id=com.nutomic.syncthingandroid&hl=en_US
https://en.wikipedia.org/wiki/Syncthing