Need to workout formulas from online calculators

I have identified some calculators which I need to understand the formula they are using to get the results from the data inserted into the relevant fields.

The calculators are:

Investment Calculator:
https://www.direct.aviva.co.uk/myfuture/InvestmentCalculator - all options

Pension Calculators:
https://www.hl.co.uk/pensions/interactive-calculators/pension-calculator

https://www.nutmeg.com/pension-calculator?income=50000&savings=0&return=5&inflation=2.5&fee=0.95&age=51&retirement=68

Mortgage Overpayment Calculator:
https://www.moneysavingexpert.com/mortgages/mortgage-overpayment-calculator - it is the INTEREST ONLY OPTION we need the formla to.

Retirement Income Estimator:
https://www.fidelity.co.uk/retirement/retirement-income-estimator

Loan Repayment:
https://www.moneysupermarket.com/loans/calculator/ - both options.