Hi, adamgondek.
If we understand correctly, you’re wanting to combine the content you currently have on an Android and the content on an iPhone onto one iPhone 12, but it’s important that you’re able to keep the content separate. If we’re on the same page, we’re happy to see if some of our suggestions will help.
The best way to do this, is to restore the backup from your old iPhone to the new iPhone first. Otherwise, if you restore the backup from the old iPhone to the new iPhone and then try to move your data from the Android phone to the new iPhone, it is just going to overwrite everything that you transferred from the old iPhone. This article can help with those steps.
Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch
After you restore your backup from your old iPhone to your new iPhone, then you'd need to manually transfer your data from the Android phone.
We encourage you to bookmark this page to use when you’re ready to transfer:
Move from Android to iPhone, iPad, or iPod touch - Apple Support
You can go ahead and set your new iPhone 12 up with your iPhone 8 backup, make sure both numbers are working correctly, and then move your Android content. Also, take a look at the section titled "Use two phone numbers for calls, messages, and data" in the following article, if you need help with using a Dual SIM.
Using Dual SIM with an eSIM
We appreciate you taking the time to post in the Apple Support Communities.
Warmest Regards.
