A: Power BI is a business analytics service by Microsoft. It aims to provide interactive visualizations and business intelligence capabilities with an interface simple enough for end users to create their own reports and dashboards. It is part of the Microsoft Power Platform.
D: Power BI Apps are an easy way for designers to share different types of content at one time. App designers create the dashboards and reports and bundle them together into an app. The designers then share or publish the app to a location where you, the business user, can access it. Because related dashboards and reports are bundled together, it's easier for you to find and install in both the Power BI service (https://powerbi.com) and on your mobile device. After you install an app, you don't have to remember the names of a lot of different dashboards or reports because they're all together in one app, in your browser or on your mobile device.
Reference:
https://docs.microsoft.com/en-us/power-bi/consumer/end-user-apps