The bets are determined by checking a wide variety of projections against my own knowledge of how players will be utilized given game context. The player prop lines were taken from this site.
When analyzing projections from fantasy sites, you'll often find that the projections are higher than betting lines, which will make you bet shit tons of overs. This is a common mistake. The projections are mean based, which tend to skew higher because the range of outcomes are not evenly distributed. (i.e., a running back on a good day could go for over 100 yards, but he would never go under 0... hopefully). If you find a mean projection lower than the Vegas line, the under is likely a good bet. To bet an over, the mean projection should be significantly higher. All bets below will follow that logic.
San Francisco @ Philadelphia
Deebo Samuel u21.5 rushing yards -114 (FanDuel): Samuel is projected around 16.8 yards. He hasn't seen more then 4 carries in a game since week 3. Even if he gets 4 attempts (realistic projections are at 3.0), he'd need to average 5.375 yards per attempt to get there. He's also gone under this number in 9/15 games this season.
DeVonta Smith u66.5 receiving yards -109 (Caeasars): Smith is projected slightly below this. He'll have his work cut out for him against a 49ers defense that ranks 5th in pass defense DVOA.
AJ Brown u4.5 receptions +135 (DraftKings): I've seen projections (PFF) which have Brown at exactly 4.5. I think those projections are a bit too low, but I can't turn down +135 here.
Cincinnati @ Kansas City
Joe Burrow u17.5 rushing yards +100 (DraftKings): Burrow has gone under this number in 11/18 games this season and is projected around 15 yards. I'll happily take this at even money.
Joe Mixon u60.5 rushing yards -115 (PointBet): Mixon is projected in the low 50s. Even though Kansas City's defense has a reputation for being awful against the run, they've been far better this season, ranking 15th in run defense DVOA.
Kadarius Toney u35.5 receiving yards -114 (FanDuel): This number is too high, Toney is projected in the high 20s.