The Chrome User Experience Report (CrUX) data is collected from real browsers around the world, based on certain browser options which determine user eligibility. A set of dimensions and metrics are collected which allow site owners to determine how users experience their sites.
The data collected by CrUX is available publicly through a number of tools and is used by Google Search to inform the page experience ranking factor. The CrUX data for the last 28-days is made available each day (with a two day lag). A more permanent BigQuery monthly snapshot is released on the second Tuesday of each month for the previous month.
Not all origins or pages are represented in the dataset. There are separate eligibility criteria for origins and pages, primarily that they must be publicly discoverable and there must be a large enough number of visitors in order to create a statistically significant dataset.