Well, via the 1090MHz extended squitter which is used by Mode S as well. Therefore usually ADS-B is closely related to Mode S. For example europe requires Mode S enhanced surveillance which in itself is very close to ADS-B out, it just misses the aircraft position. Most Mode S enhanced surveillance installations send that data additionally and therefore fulfill ADS-B out although it is not currently required in europe.
You could try the
Eurocontrol CASCADE website which should have quite a bit of information about ADS-B.
Of course you could take the IANS
course about ADS, or take a look at the pre course.