What is the difference between puri.sm vs Google regarding privacy? Won't user data end up on one of their servers?
Can I self host the phone ecosystem? (For example having my own app "store" or maps)
The best way to think of it is that Google is an identification service.
They make their money by identifying who you are, and creating a datastore with as much information about you and your activity as possible. This is both online data and offline data. Online data is collected directly via google services and indirectly services like google analytics, google javascript libraries, google fonts, recaptcha and more. Offline data is also purchased and stored, for example consumer credit card purchase data.
Given that, it is clear that every free service they offer can be funded by data collection. Are your kids using google docs? Do your friends email you from gmail? do your coworkers use google sheets?
Ok, so purism doesn't make its money from data. It makes its money from selling you a phone. This is a lot less money than google makes, because you pay it directly to them. They can't turn around and get millions from advertisers.
On the other hand, because data is not their revenue stream, they don't care if you store your mail on their servers or on your own or where you get apps or even what OS you run. So there's no need for dark patterns to get you to interact with them.
Yes, you could have your own store. You could have your own OS. You can create and maintain and share your own maps or data or whatever. Actually linux people have been doing this for years and years. check out nextcloud.
A lot depends on what online services you are using. The goal of Purism is to provide a suite of online services at Librem One (https://librem.one/) so you don't have to share your personal data with Google, Facebook, Twitter, etc.