Hi Rash K,
The tax free allowance for this tax year Apr 24 - Mar 25 is 12570, which allows your to earn £1047.50 every month before you start paying tax.This is what is being applied to your placement employment (1257L code) , if you commence at 2nd P/T employment because, all you personal allowance is used at the main job, you would be taxed at the Basis Rate of tax which is 20% which will be shown as a BR code. At your 2nd job your should sign a P46 with them and tick statement c to indicate that it is not your main employment.
Income Tax rates and Personal Allowances /
Tax Codes.
Thank you.