I didn't know that. I had someone buy my iPhone and the store set it up so it says "Customerphone" and have no idea about any apple account other then my pin. When I got my last one they had me hook it to computer and set it all up since I couldn't see the screen and turned my new iPhone 7 back into the iphone6 that I upgraded from. I lost some of the new apps which seems dumb to me. Why would you buy a new phone and have the features of the old one?
You don't, and you didn't. You can't downgrade an iOS device to an older version of iOS, let alone one from another model. When you migrate from one device to another (which is, at its core, a backup to either iCloud or iTunes, and then a restore from that backup to another device), the OS on the new device remains as it was, and your user data is copied, together with settings, and apps are reinstalled. The only apps you can lose in such a migration are ones that don't run on the new device, if the latter is running a newer OS version and the old apps haven't been upgraded to support the new OS. Also, if on the old phone you deleted some of the default iOS apps, then on the new one you can still return to the App Store and reinstall them.
How do you find this out, or prevent this?
There's nothing to prevent. Whatever happened isn't what you think happened.
Luckily for me I just found a book with half my passwords in it. Still don't have the account info for my binance account. More lost bitcoins.
What does this have to do with your iPhone upgrade?!
Can you make an encrypted or otherwise secure word file with your passwords then somehow store in gmail/ google drive? so no matter where you are you always have a master list of passwords. I used to do this with my resume and got sponsored for a stock brokers license by having a resume I printed off the nearest computer. This was when the internet was still young so they were quite impressed I could just produce a resume on the spot.
Of course you can do this, but using proper password manager software is a vastly wiser approach than a Word or Excel document. (Remember that the password protection in Office is trivial to circumvent, since it doesn't actually encrypt the data within.)