There are two forms of the TOEFL today: the paper-based test (PBT) and the internet-based test (iBT). In most countries, the iBT is by far the most common test form, so that’s the form we focus on.
The TOEFL iBT is formatted in four sections, each of which tests one language skill. The test isn’t adaptive, which means the questions don’t get harder if you do really well or easier if you get a few questions wrong. Multiple forms of the test exist and they change often, which helps to prevent cheating, but all of the tests are close to equally difficult.