It is impossible to create detector for LLM generated writing. The LLM detector technology must beat the LLM at its own game and if it can we can easily create a better LLM. Anyone who claims they can do it is sorely mistaken or flat out lying.
And of course you don't tell people how your LLM detector works, don't provide a public API, and preferably leave it ambiguous whether it even exists. That solves most issues with LLMs using your detection method to get better in future versions.
That's counter to the business model of these LLM detection websites, but Google doesn't have a problem with keeping any potentially existing models inhouse and locked down.
If you detect LLMs 100% of the time correctly, it is irrelevant if you also incorrectly detect human written text as LLM text 80% of the time. "Detect them all and let god sort them out" will eventually fail as a business plan.