Hi flypig,
Here is documentation explanation about their relationship:
Dynamics 365 apps, such as Dynamics 365 Sales, Dynamics 365 Customer Service, Dynamics 365 Marketing also use the underlying data platform (Dataverse) used by Power Apps to store and secure data. This enables you to build apps using Power Apps and Dataverse directly against your core business data already used within Dynamics 365 without the need for integration.
https://docs.microsoft.com/en-us/powerapps/powerapps-overview#power-apps-and-dynamics-365
In a word, Dynamics 365 can be regarded as a group of apps built by Power App, and Power App provides some connectors for users to connect their Dynamics 365 to other data sources.(It is a two-way process.)
The article and the graph might help you to understand the relationship between Power App and Dynamics 365:
https://readyxrm.blog/2020/06/05/you-likely-dont-need-dynamics-crm-you-need-a-power-app/

So let's back to your specific question: even though for Dynamics 365 Marketing, it is still a Power App built app.(Of course there are some dedicated solutions and components developed by MS).
Therefore, due to this fact, PowerApps could still be your option to connect to your oracle database, just like we customize other Dynamics 365 apps.
However, it is still possible to connect to other DBs using Plug-in, and it can be regarded as traditional and secondary development, e.g: ClickDimensions, a third party solution provider who extends marketing module with their customization before the release of Dynamics 365 for Marketing app.
While Flow or other PowerApps tool such as on-premises data gateway are the recommended modern way because Dynamics 365 is now running on Cloud, heavy code task is less required to customization nowadays.
Regards,
Clofly