It’s 1 and a half year, and the reason it exists is (a) most students graduate and start jobs after the current year’s H1B process is already over, and (b) if you don’t get your employment visa in the first round (since it’s capped at 60k), then they want to give you a second chance.
> Only if your employer has already applied for your H1B. Doesn't seem like so to me, the requirements for the extension are (from [1]): >> Have a degree in an eligible STEM field from a Student and Exchange Visitor Program-certified school that is accredited when the student submits their STEM OPT extension application to USCIS. >> Pursue their STEM OPT extension through an employer that is enrolled in USCIS's E-Verify employment eligibility verification program. >> Select a STEM OPT employer that provides the student with formal training and learning objectives. Work a minimum of 20 hours per week per employer.
[1]: https://studyinthestates.dhs.gov/stem-opt-extension-overview