Explanation:

Box 1: Custom self-service portal for both employers and job candidates If you select an environment that contains customer engagement, you can create the following portals:
Customer self-service portal: A customer self-service portal enables customers to access self-service knowledge, support resources, view the progress of their cases, and provide feedback.
Partner portal: A partner portal allows every organization with resellers, distributors, suppliers, or partners to have real-time access to every stage of shared activities.
Employee self-service portal: An employee self-service portal creates an efficient and well-informed workforce by streamlining common tasks and empowering every employee with a definitive source of knowledge.
Box 2: Model-driven app with push notifications
Compared to canvas apps, model-driven apps in PowerApps are based on underlying data - specifically, the data stored in Common Data Service (CDS).
Box 3: Webform with target set to the job custom entity
Box 4: Configure a value for the Execute Workflow on Redeeming Contact option only.
Execute Workflow on Redeeming Contact: A workflow process to be executed when the invite is redeemed.
The workflow will be passed the redeeming contact as the primary entity.
Box 5: Configure the value for the Assigned to Account option only.
Assign to Account: An account record to be associated as the redeeming contact's parent customer when the invite is redeemed.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/portals/portal-templates
https://global.hitachi-solutions.com/blog/canvas-vs-model-driven-apps
https://docs.microsoft.com/en-us/powerapps/maker/portals/configure/invite-contacts#invitation-attributes

If all the entities in external data source have an associated GUID primary key then we can implement the virtual entities for sure. For Reference: https://learn.microsoft.com/en-us/dynamics365/customerengagement
/on-premises/developer/virtual-entities/get-started-ve?view=op-9-1

Use LoadData and SaveData for basic data storage while offline.
Note:
When building mobile apps, one of the most common scenarios app makers face is how to enable their users be productive in situations where there is limited or no connectivity at all. This has been one of the most requested features for PowerApps to allow running apps while being disconnected and to provide some support for offline data caching. In this release of PowerApps, we are delivering the first set of improvements for app makers to achieve that by enabling:
Launching the PowerApps mobile player app offline
Running apps while being offline
Determine when your app is online or offline or in a metered connection by using the Connection signal object.
Leverage existing formulas such as LoadData and SaveData for basic data storage while offline.
Reference:
https://powerapps.microsoft.com/sv-se/blog/build-offline-apps-with-new-powerapps-capabilities/

Two security models can be used for hierarchies, the Manager hierarchy and the Position hierarchy. The Position hierarchy allows data access across business units. If you are a financial organization, you may prefer the Manager hierarchy model, to prevent managers' accessing data outside of their business units. However, if you are a part of a customer service organization and want the managers to access service cases handled in different business units, the Position hierarchy may work better for you.
Note: The hierarchy security model is an extension to the existing security models that use business units, security roles, sharing, and teams. It can be used in conjunction with all other existing security models. The hierarchy security offers a more granular access to records for an organization and helps to bring the maintenance costs down.
Reference:
https://docs.microsoft.com/en-us/power-platform/admin/hierarchy-security