Birkat Keriat Shema

Question: I understand that at a minyan, the chazzan is required to repeat Shmoneh Esreh out loud so that people who may not know how to daven can fulfill their obligation to daven with the chazzan’s repetition. What, however, should the chazzan do when he reaches Kedushahh and Modim? I hear some chazzanim say every word of Kedushahh out loud and some only say the last part of the middle two phrases out loud. As far as the congregation is concerned, I hear some congregants say every word of Kedushahh and some say only the last part. Finally, some chazzanim and congregants say Modim during chazaras hashatz out loud and some say it quietly. What is the source for these various practices? A Devoted Reader (Via E-Mail)