with + instrument/material/ingredient
The dam was built with steel and concrete.
by + agent
The organisation was started by the government.
Note: We omit by when who/what did the action is not known or not important.
1 The wallabies were brought to Scotland by Lady Colquhoun.
2 The hikers were equipped with raincoats and walking boots.
3 Over the years, the forest has been destroyed by people cutting down trees.
4 The Kazakh steppe is covered with grass.
5 The red panda is hunted by people for its fur.