I'm sure there are plenty of resources on this topic, but I don't know what keywords to use to find them.
I want to know how you might create a noun by saying that the noun does something.
I saw the cow that ate vegetables at the shop
Would be something like
The bracketed part of the above sentence is the clause that I don't know how to turn into a noun.
Note that I'm not looking for a sentence like
That would mean
I saw the cow eating vegetables at the shop
Because the cow did not eat the vegetables while I saw it, that's just how I'm identifying the cow.
Hopefully, the question is clear. Thanks in advance.
Edit Even a name for what this is called so that I can do further reading would be invaluable.