They kept silent while they were listening to Paul and Barnabas speak. So the discussion wasn’t over based solely on what Peter said, Paul, Barnabas, and James all supported what Peter said after he spoke. In the end it was James who made the final ruling hence his words in verses 19 and 20
“Therefore it is my judgment that we do not trouble those who are turning to God from among the Gentiles, but that we write to them that they abstain from things contaminated by idols and from fornication and from what is strangled and from blood.”
Acts 15:19-20 NASB1995
James made the final ruling and decided the next course of action.
The Council of Jerusalem (Acts 15) gathered to address this problem:
"some of the sect of the Pharisees who believed rose up, saying, "It is necessary to circumcise them, and to command them to keep the law of Moses.""
Notice that when Barnabus and Paul spoke, and then James, neither of them said a word about circumcision or the law of Moses. That's because Peter had already settled that issue with his speech, which put an end to the debate, which is why the assembly "fell silent" (v.12).
Barnabus and Paul talked about "how many miracles and wonders God had worked through them among the Gentiles".
Then James added "that we write to them to abstain from things polluted by idols, from sexual immorality, from things strangled, and from blood (v.20).
The subsequent letter sent from the Church to the Gentile believers included:
(a) Peter's decision regarding cirumcision and the law:
"Since we have heard that some who went out from us have troubled you with words, unsettling your souls, saying, "You must be circumcised and keep the law"--to whom we gave no such commandment" (v.24)
and
(b) what James added:
"that you abstain from things offered to idols, from blood, from things strangled, and from sexual immorality" (v.29).
In other words, it was PETER - not James - who settled the issue that the Council had gathered for.