Short answers, pulled from the story.
Frodo Baggins, Samwise Gamgee, Meriadoc Brandybuck, and Peregrin Took arrive together at Brandywine Bridge. These four companions face a local police force called Shiriffs under Saruman's rule.
The Battle of Bywater occurs the next morning after the hobbits discover Sandyman's mill has been replaced by a noisy machine. Merry blows a magic horn given to him by Éowyn of Rohan before the fighting begins.
Lotho Sackville-Baggins moved into Frodo's home Bag End when Frodo left on quest. He became known as Chief or Boss and used unexplained funds to pay for wagonloads of goods including tobacco sent away from the Shire.
Tolkien wrote The Scouring of the Shire with environmental themes based on his childhood experiences in Sarehole rural Warwickshire taken over by growing city Birmingham early 1900s. Old mills fell into disrepair while trees were wantonly destroyed without reason.
Pippin returned with one hundred family members to rally the large Took clan during The Scouring of the Shire. This group arrived just before a larger force reached the village to defeat the ruffians.