How can a sock gets into a washer's drain boot
The washing machine consists of what is effectively a barrel with a spinning cage inside.  That cage is where the clothes go.  There is a gap between the top of the cage and the barrel.

If the gap between the top of the cage and barrel were big enough, and the washer were carelessly loaded, then a small item could easily fall behind the cage into the barrel, where it would get sucked into the drains.