Placements aren't affected by KDA at all, but instead are based on wins/losses and the ranks of who you played vs.
Most new player's fresh accounts will land around silver 3 going 50/50 in placements.
Every year when the season resets, your current rank will get reset and your MMR will get soft reset so that next year's climb can take place (this will happen in January) most people can expect to place a bit lower than they were, so it's normal for Silver 3 to then place Silver 4 or 5 at the start of the next season.
If your friends are B1/2 and only climb to S5 each season, then yes, they will keep placing B1/2 every year. They need to climb past that to place higher.