Business Units vs Access to records

Ok, I have a (probably really dumb) question.

We are looking at using Business Units in CRM since we just acquired a new company. It needs to be in the same CRM, but with different users accessing different data.

I understand the Business Unit concept and how it is related to Users and Roles.

What I don't get is : How a specific record is associated to a Business Unit?

Whether it's an account, contact, opportunity, etc.

What makes Account A part of Business Unit A and Account B part of Business Unit B?

Is it the owner of the record?