We do not systematically process all the catalogues in Simbad, but the CDS also provides the VizieR database which contains published lists of objects, as well as most very large surveys.
Though in the past catalogues like 1RXS or IRAS have been enterely put in Simbad, we do not do that anymore, because the number of objects is exponentially increasing, and because usually only a small percentage of a whole catalogue is actually studied in the literature. Now, we do not want to put all SDSS or 2MASS objects in SIMBAD because SIMBAD focus on the objects of interest.
You can also perform your own cross-match using the dedicated and fast tool : CDS cross-match service.
The cross-identification between the SIMBAD database and the ESA Gaia mission Data Release 2 has been successfully performed on June 30, 2018, on about 4.5 million objects.
The following data have been upgraded in SIMBAD :
4.5 million coordinates, Gaia DR2 identifiers, G magnitudes 3.8 million parallaxes 3.8 million proper motions 2.0 million radial velocities
Objects must have had sub-arcsecond coordinates already, corresponding to quality A, B, and C in SIMBAD. Objects with insufficient astrometry, quality D or E, have been completely ignored.
Two special operations of cross-identification between the content of SIMBAD and Gaia E/DR3 have been performed in February and May 2022, involving about 7 million objects.
The operation was limited to objects with a good astrometry (coordinates with a quality D and E excluded). Note that the identifiers and astrometric parameters (coordinates, proper motions, parallaxe) are the same in Gaia EDR3 and DR3; on another hand the G magnitudes have been slighty corrected in Gaia DR3, and Gaia DR3 published about 23 million radial velocities.
The radial velocities in Gaia EDR3 are those of Gaia DR2.