One early confusion point for new Salesforce learners is when they first log in. Most Trailhead Trails and many of my own lectures are geared toward Salesforce Classic, yet when you first log in to a free Salesforce Developer or Trailhead account, you default to Lightning Experience. What Salesforce should do at this point is to pop-up a default layout option for a new user, so they can select and preview their default interface, along with instructions on how to switch back and forth.
The ultimate goal would be to get Lightning Experience where users don't have to switch back to Classic so much, but I digress. Since I get this one question so much regarding how to switch between the two interfaces, I decided to put this info very early in my upcoming Salesforce Sales Cloud Consultant Certification course.