“who stopped them?”. There are no checkpoints between Oregon and California

.. actually, there are checkpoints along every major roadway into California that do check incoming vehicles, mostly to find and prevent invasive species from entering the state and affecting the agricultural industry, and more broadly to protect the environmental systems in California. There have, in fact, been legal challenges against these checkpoints for violating travel laws, but the checkpoints have remained. They've also been used to seize non-agricultural items like weed, weapons smuggling, etc. so emissions standards checking isn't completely out of the realm.

Obviously, she's lying and she doesn't know any of this either anyway. But there are checkpoints to enter California.

https://en.wikipedia.org/wiki/California_Border_Protection_Stations