Short answers, pulled from the story.
Southwark Cathedral was designated a cathedral in 1905 when the Church of England created the Diocese of Southwark. Before that it had functioned as a parish church for more than three centuries after the dissolution of Southwark Priory in 1540.
John Harvard was baptised in Southwark Cathedral on the 29th of November 1607. He later helped found Harvard University in New England, which is named after him. His father Robert was a local butcher and inn-holder who was a business associate of Shakespeare's family.
Southwark Cathedral is the burial place of the poet John Gower, the dramatists John Fletcher and Philip Massinger, and Edmund Shakespeare, brother of William Shakespeare. Lancelot Andrewes, Bishop of Winchester and contributor to the Authorized Version of the Bible, is also interred there.
Doorkins Magnificat was a brown female stray cat who arrived at Southwark Cathedral in 2008 and became its resident cat. Dean Colin Slee named her as a reference to the atheist Richard Dawkins. She met the Mayor of London and Queen Elizabeth II, was commemorated with a stone gargoyle in 2018, and died on the 2nd of October 2020.
The main railway viaduct connecting London Bridge station to Blackfriars, Cannon Street, and Charing Cross stations passes just eighteen metres from the southeast corner of the cathedral. When the viaduct was extended in 1852, the alternative was to demolish the cathedral entirely to allow a more direct rail route.
The altar screen was erected between 1520 and 1528 during the episcopate of Richard Fox, Bishop of Winchester. It stands about thirty feet high and occupies the entire east wall below the window. The original stonework is of Caen and firestone, and it includes carved jokes referencing Bishop Fox, including a figure chasing a fox in the door spandrels.