You can find many explanations of theoretically how GDP is measured. But how is it done in practice? Is it estimated by extrapolating from small samples of activity or can the overall value be measured directly somehow.
Beyond what is written in this Economist article, the general principle is an estimation based on statistical data. The 4 major aspects are private consumption, government consumption, investment and net exports.
I've worked as an auditor and cooperated with colleagues from the national bank of Portugal, which collected and cross-checked financial data to be able to calculate the GDP. Looking at the individual components:
- The private consumption can be assessed by surveying individuals, making assumptions in connection with the tax information and financial aggregated data from banks (who are obliged to report to the National Banks)
- Government consumption is publicly available
- Investments are estimated by looking at company data and company tax filings
- Exports and Imports are calculated by crossing the info on private consumption, investments, and bank data on payments sent overseas