Who holds authority in it, and how is that authority organized?

Authority belongs to God's word — the Qur'an and the authentic Sunnah of His Messenger ﷺ — not to any person or office; every dispute is taken back to them (Qur'an 4:59). There is no clergy and no priesthood: anyone may call on God directly (Qur'an 2:186). Scholars explain the texts and the four schools of law organize that work, but a scholar's word weighs only as much as his evidence.

None; loose intellectual networks.

None; an individual epistemic stance, not a community.

Varies: papal (Catholic), synodal (Orthodox), congregational (Protestant).

Rabbis (teachers/interpreters); no central religious authority.

Sangha (monastic community); teachers, lamas (Tibetan), roshis (Zen).

No central authority; gurus, swamis, schools (sampradayas).

Granthis read scripture; community decisions via Sangat; no clergy.

Secular state, courts, neutral civic institutions.

Humanist associations (IHEU, American Humanist Association).

Constitutional governments, rule of law, free institutions.

Decentralized; academic, activist, and political networks.

The nation-state, political leaders, cultural institutions.