When St Paul says all churches he means all parishes. All such gatherings. Not alternative jurisdictions which profess wildly unChristian things. Paul was clear to be mindful of such wolves.
Christ founded and built His Church (singular) on Peter's Confession. When Paul wrote of churches in the plural he was referring to parishes or in today's world, individual parish churches (obviously in Paul's time, it was not possible for the parish to have Mass in the same location every week).