If it's native, you should have totally nothing to care about if you are using. The tabbar is supposed to be announced like so to the screenreader while taping it or sliding the finger over it, and each items within the tabbar should be announced as a key entry to your navigation.
If you are building an app using React Native for instance, I found some documentation about it http://semantic-portal.net/react-native-guides-accessibility but that's a technical aspect for implementation, not really a UX issue.
Best is to use what the OS give you because Android and iOS are quite good by default in term of accessibility. If you rebuild things via a framework, there is always a way to make it accessible, like :
- Putting the menu (tabbar) before the main content so a linear screenreader (reading the source code and not reading what's the user is taping) will read it before the long content.
- Using semantic elements instead of neutral element.
I think that's all for the main idea. Most of the rest is implementation that depends on your development context.
Hope I could help at some point.