coordinates in ICRF - Astronomy

The International Celestial Reference Frame (ICRF) is defined based on distant quasars and the origin is the barycentre of the Solar system. So, how can we measure the coordinates (alpha, delta) of an object, say, star in ICRF. In web, the coordinates are often reported as (alpha, delta) in ICRF analogous to (alpha, delta) in equatorial coordinate system. And what role does distant quasars playing here.
I know how the coordinates are defined in equatorial coordinate system.

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