Set up a default app per profile in Lightning Experience

iTo set a Lightning App as the default app when users login: 

If you are using the Standard Profile User Interface:

1. From Setup, enter Profiles in the Quick Find box, then select Profiles
2. Select a profile and scroll to the 'Custom App Settings' section.
3. Select Default next to the Lightning app.
4. Log out and log in again.

If you are using the Enhanced Profile User Interface:

1. From Setup, enter Profiles in the Quick Find box, then select Profiles.
2. Select a profile and click Assigned App.
3. Click Edit.
4. Select Default next to the Lightning app.
5. Click Save.
6. Log out and log in again.

Note: 
1. To learn how to Enable the App Launcher with a Profile in Salesforce Classic.
2. Assigning a default app will only allow Users to see the app when they login to Salesforce for the first time. However, when a user switches apps during their session and logs back in, they are taken back to the last app they used. Please upvote on this Idea: Default App (Landing App)