How is a congregational prayer completed according to the Maliki school for someone who missed a Raka'ah that was to be recited aloud: by reciting Al-Fatihah silently or by reciting Al-Fatihah and a Surah aloud?

You are required to make up the missed رکعہ by reciting Surat Al-Fatiḥah and a surah aloud. For what one catches up with the Imam is considered the beginning of one's prayer in terms of standing and sitting, not in terms of recitation.