It will be field data. In their announcement of the algorithm update, Google are clear about using "real-world, user-centered metrics". By definition, lab measurements are not real-world, and what I see in my lab might not be the same as what you see in yours.
Google themselves are clear on this in their documentation for speed tools: lab data "might not capture real-world bottlenecks" and "cannot correlate against real-world page KPIs".
The Web Vitals documentation is also clear on this point (emphasis mine):
"While all of the Core Web Vitals are, first and foremost, field metrics, many of them are also measurable in the lab."
The data used will almost certainly come from their CrUX (Chrome User Experience Report), since it "provides user experience metrics for how real-world Chrome users experience popular destinations on the web".
Edit: This is now confirmed by Google.