Start a new design and describe what you need — e.g. "a wall bracket for a 60 mm fan, mounted with M4 screws". The AI asks only what it must, then builds a real parametric model.
Or upload an STL and describe a change — "make a hole in the middle, 3 cm diameter". STL edits are mesh operations: holes, cuts, added blocks, scaling — but no fillets or shells, and no Onshape model (a mesh isn't parametric).