In the upper-left corner of the first panel, there is a drop-down menu labeled Operations.
In Operations mode, you can issue queries or mutations to the API. However, if you want to look at the API definition, change this to Schema Reference, which shows a list of queries. You start in the Query object, but you can use the Types panel on the right to switch to the Mutation object.
Note that once you drill into an object on the left, it is not possible to jump back out. You need to use the Types panel on the right to navigate. If you get lost, click the Back Arrow, and then click Query to get back to the Query object.
