I've never had a job paying less than the NI lower limit, but I get a retrospective tax calculation that says (income from A, income from B, total income, total tax paid, total tax due, NI paid, NI due) and usually a small refund of overpayments.
The tax allowance I believe is usually granted by tax code to your "main" source if on PAYE and the other ones some kind of estimate. But until after the end of the year, when Actual figures are known, there will be some estimating of your band and deductions. I still prefer PAYE and keeping inside allowances because it saves the effort and cost of preparing a tax return and possibly dealing with queries.
For some foolish government reason the NI band limits are not the same as tax bands, and I believe pension and benefits amounts change at a different date, just to make the calculations more complicated. I'm told that on Universal Credit, people so often get paid the wrong amount, and are subsequently ordered to repay it, that some of them can't bear the stress and stop claiming. People on benefits often have so little money that they don't have savings and can't afford to live on air if a benefit is suddenly stopped or reversed. I recall Iain Smith saying that people on benefits would have to learn to budget better....
