@TMwd record ownership in HubSpot is entirely user based, see here: Assign ownership of records -- assignment is not happening randomly, you have full control over it. If you’re unsure how it’s happening, check the property history if the owner field on a record.
Records cannot be assigned to teams directly (aside from manually sharing a record). The team if a record is just a reflection of the team the user is in.
Whether someone can access a record depends on whether it’s assigned to them (owned only) or by someone in their team (team-only). Membership in multiple tams can easily mean that a user can effectively see all records, via shared team memberships.
(For exact instructions, I would have to better understand what internal and external sales people should be able to do and access, and examples that illustrate your team structure.)