quarantine 'em

I'm watching Back to the Future II, where the main characters travel four years into the past,

So they don't have smartphones but headsets are popular - although when someone gets a call they all ring and display PHONE on the outside?

Also all their video feeds are janky. Fax is ubiquitous (lol) and video calls end with a "thanks for using AT&T" ad roll

Ok so (spoilers ahead) the plot rests on Biff winning a lot of bets because of a sports Almanac from the 2000s. Then he lobbies for gambling to be legal? Let me tell you if someone reliably won they wouldn't just sit back and take the losses (why yes I *am* fun at parties)

Did you know the director of Back To The Future started a rumour that hoverboards were real but they weren't marketed because of parents groups concerns about safety?

Ok so re,: time travel, I get that old Biff came back in time to give the Almanac to young Biff. What I don't get is why Marty and Doc landed in the "Biff is Trump" timeline, not in their own timeline. What determines which ancestor timeline you go back to?

I need to find a good resource about the different time travel models that goes in excruciating detail