In Dynamics 365, there are applications running on the legacy Web Client and those running on the Unified Interface (or Unified Client Interface). There is a way to default users to only run apps on the Unified Interface.
To do this, go to System Settings and find the setting Use the new Unified Interface only (recommended):
Now, when you go to your Dynamics 365 url, you will see a list of your apps. You may be directed to a url something like https://yourorg.crm.dynamics.com/main.aspx?forceUCI=1&pagetype=apps:
Selecting a Web client app will take the user directly to the Unified Client app:
To get back to System Settings, go to Settings->Advanced Settings:
And navigate back to System Settings: