If you have more than one job, each employer should be asked to start making deductions by HMRC. The amount they deduct will depend upon your level of income.
Each employer will apply the annual threshold in computing deductions and will be unaware of your other earnings.
Exceptionally, if you have more than one job with the same employer, and, your income is combined for NIC purposes, the employer will do the same for Student Loan repayment purposes.
Note: If the borrower submits an SA return then all income from the various sources will be totalled and an additional payment of Student Loan repayments may be demanded. In other cases each employment is looked at separately.