Dynamics 365 has an online marketplace, AppSource, where you can search for apps to install. These apps are written by vendors and also by Microsoft. It has products written for Cloud Solutions, Power BI and Office 365 as well as Dynamics 365.
To use AppSource, go to https://appsource.microsoft.com/ and sign in with your 365 credentials.
Notice the products you can filter on at this point.
Scrolling down you will see the top app results and also the top partner results. As you select above it will filter the results below:
Selecting on See All, you get to drill down even further into the results:
You can then finally select an app and it will give you information on what the app is, what platform it runs on and if you would like to install it: