Tax on 2 jobs - one full time, one zero hour contract

So I am looking to start a new job soon, which will be my main source of income. I will be working 40 hours a week on minimum wage, and I'm 21 so that should be £10.18 per hour, so looking at £19-21k a year, depending on how you work it out, and I know that will be my main source of income that I will get taxed on. However, I am also looking to start back somewhere where I used to work, but only on a 0 hour's contract, and maybe pick up an extra 5-10 hours a month on this job. If I understand right, will I remain in the 20% tax bracket, and will my tax come off both payslips or just my main job's payslip? Also, regardless of whether I do any work for that second job or not some months, will I always pay more tax being employed by 2 workplaces, than if I was just to employed by one place of work? So for example, say one month I earned £1600 in the main job, and £200 in the second job, will I get taxed 20% of £1800? But another month I just earned £1600 in the main job, and did no work for the second job, would it be 20% of £1600 or would I permanently get taxed as if I was earning £1800? Any answers would be very helpful as it is quite confusing. Thank you!

Hi
The tax is based on your income rather than how many jobs you have.
If you earn above the Personal Allowance on the main job, you will pay tax on the income above the Personal Allowance at this job and 20% tax on the second job.
Thankyou.