Microsoft has released some good material about their Project products, so here it go:
This article gives you an overview of the logical architecture that exists for key apps that are provided in Project.
Key Project apps
The architecture diagram above shows the key apps that are available through Project Plan subscriptions:
Project for the web
Project Online Desktop Client
For more details about features that are available through these project apps and services, see Feature availability across apps and subscriptions in the Microsoft Project service description.
The key Project apps described in this article are available in the following Project Plans.
Note: In Project Plan 1, users can only view roadmaps in read-only.
For more details on Project Plans, see the Microsoft Project Service Description.
Project for the web
Project for the web provides simple, powerful work management capabilities to meet most needs and roles. Project managers and team members can use Project for the web to plan and manage work of any size.
Project for the web is built on the Microsoft Power Platform. The Power Platform consists of PowerApps, Power Automate, Power BI, and the Common Data Service (CDS). Integration with the Microsoft Power Platform lets you easily leverage its components to create custom business solutions and do advanced analytics and reporting on project data.
Just like Project Online, users can access their Project for the web projects through the Project Home page. It will by default list projects that were recently viewed, owned by, or shared with the user.
Project for the web data is saved to the Common Data Service (CDS). CDS is part of the Microsoft Power Platform, which Project for the web is built on.
When Project for the web or Roadmap are first used in an Office 365 tenant, a Default CDS instance is provisioned for the tenant. Project for the web data is saved as entities to Solutions that are in the default CDS instance, and can be identified by their display name of Project.
You can use PowerBI Desktop to import and analyze not only your Project Online data, but also your Project for the web data as well. You can use the same Project Power BI template to view a Portfolio dashboard of reports that can be helpful in analyzing your data.
Use Roadmap to create a collective view of projects that are important to you. Your roadmap can connect to projects created in multiple tools, such as Project Online, Project for the web, and Azure DevOps.
As mentioned previously, Project for the web and Roadmap data are saved to solutions in the default CDS instance. While Project for the web data is saved as entities in Project solutions, Roadmap data is saved to entities in CDS solutions that have a display name of Portfolio Service.
For the full article, you can access here: https://docs.microsoft.com/en-us/project-for-the-web/project-architecture-overview