Got it. So you're aiming to be a full stack framework. Let's say I want to keep all my business logic and schema in Rails. But I want a UI for web + native, would One work in this case? It is kind of where my lack of knowledge about how Zero works is causing my confusion on all this.
You definitely can, so long as you use Postgres you should be able to use Zero (as of now they have a few limits on column data-types as well).
Also - One is not tied to Zero, you can use One for just about any web/native project and bring any libraries you want, we just are making it nice if you follow a certain path.
Basically Zero will run in the browser and has it's own server which will exist next your database. So you can do all your database management through Rails and there is a section about Custom Mutators where if you need to pipe data through to AR to have it run callbacks, that is possible.