Canvas apps are the correct choice for the "base check-in solution" because they provide a highly customizable, "pixel-perfect" user interface suitable for task-specific scenarios like a guest check-in kiosk. Unlike Model-driven apps (which are data-centric and rigid) or Portals (designed for external web access on personal devices), Canvas apps are optimized for tablet-based, front-line interactions on shared devices.

To access the solution on the check-in devices (tablets), the Power Apps mobile app is required. This application acts as the player/container that runs Canvas apps on iOS and Android devices, allowing the app to utilize native device capabilities (like cameras for scanning) and ensuring a seamless user experience. The "Dynamics 365 for phones and tablets" app is primarily for Model-driven apps and is being deprecated in favor of the unified Power Apps mobile app.

Microsoft Learn: What are canvas apps?

Quote: "Canvas apps enable you to organize the user experience in the way you want... ideal for task-based or role-based applications."

Source: https://learn.microsoft.com/en-us/power-apps/maker/canvas-apps/getting-started

Microsoft Learn: Get started with Power Apps mobile

Quote: "When you create an app, or someone shares an app with you—either a canvas app or model-driven app—you can run that app on iOS and Android devices by using Power Apps mobile."

Source: https://learn.microsoft.com/en-us/power-apps/mobile/run-powerapps-on-mobile

Microsoft Learn: Dynamics 365 for phones and tablets deprecation

Quote: "Dynamics 365 for phones and tablets... will be deprecated for online users... Online users must migrate to the Power Apps mobile app."

Source: https://learn.microsoft.com/en-us/power-platform/important-changes-coming#dynamics-365-for-phones-and-tablets-will-be-deprecated