They look like they are shoved directly into the buttercream which is holding them in place. But I bet you could use toothpicks for extra security if you want. Just put one in the strawberry and then insert it into the cake.
Don't use toothpicks...….that will make the berries leak :( Dip them in a bit of melted choco (white or brown as needed) and hold them in place for about 10-30 seconds until the melted choco starts to set up. Start at the bottom so they help hold the above/new one in place :) Look closely & you will see the 'puddle' of melted choco around each holding it.