The algorithm used to be very complex and a big trade secret. Now it's just a bunch of big neural networks, and while still secret, doesn't look very different to Bings. The difference is Google has much more data to train theirs, so the results are better.
It was an algorithm originally. Today it's a bunch of big neural networks as you say, which are basically impossible to document, much less explain changes in, or even know if anything changed.
