Hi,

You can use the odoo external api/xmlrpc to fetch and update from the odoo database.

You can refer this documentation: Odoo External API

Video tutorial: Odoo External API: Search And Read From Odoo Database

Thanks & Regards

That really depends on your use case, i.e., which models you want to access, which language you use (e.g., Flutter), etc. But see link below for the official documentation which should answer most of your questions:

https://www.odoo.com/documentation/16.0/developer/reference/external_api.html

I hope this helps!