Earthworms are well-known for loosening soil and adding fertility to it. So they both break it up, and the worm castings also hold moisture to help hold it in place so it'd be less likely to erode.
Roots help keep the soil in place.
Plant roots are the most important structure holding soil in place; of course, gravity also holds it in place, moisture makes it stick to itself to some degree, and anything on top of it will help to hold it in place.
Roots
Yes, the root system grows down into the soil forming a fibrous network that holds the soil in place.
A root holds the plant in place and it absorbs water and minerals from the soil.
A worm lives in soil.
Breaks it up
dry soil
roots
surface soil
Moss breaks up the soil and then it decomposes and fertilizes the soil
Water and or rocks