Neither, it's unethical and there's no possibility of doing that in legal way.
However I do write 1-2 hour PoCs on my spare time and my own equipment, using only publicly available stuff - they sometimes come handy at some point later. If we assume 'remote first' development is okay - with no possibility to test stuff locally, well, we're back to either bookmark managers or pet projects to keep at least a bit of knowledge between jobs.
And the new company would also be liable for using trade secrets that they shouldn’t.