tax year

I dont know whether this is silly question, but I am still confused about the “tax year”.

This is one of question: Bay is self-employed for many years, prepared account for September each year. on 1 December 2020, he employed a staff and paid 3300Pound per month. Question: Calculate NIC1 employer of new staff for tax year 20/21.
My understanding is that as he started business for many years, the tax year 2020/2021 should be from 1 October 20 to 30 Sept 2021, then the NIC should be calculated for 9 months (1 Dec to 30 September) of the year. however, the answer is for 4 months only, up to 5 April 2021.

I understand from your lectures on the basis period that we always need to calculate up to 5 April but for only the first tax year and the last tax year, all the other tax years, the trading should be assess based on the CYB…

Thank you for your help