Is the GRE section-adaptive? How does section adaptivity work?

Yes. For both Verbal and Quant, everyone starts with a first section of average difficulty. Your performance on that first section determines whether your second section in the same subject is harder (with higher scoring potential) or easier (with a lower scoring ceiling). Verbal and Quant adapt separately: your Verbal performance has no effect on your Quant sections, and vice versa.

Within each section, you can move freely between questions, and every correct answer earns the same credit.