www.makemkv.com

There is a product page for the MT1959: https://www.mediatek.com/products/optic ... ive/mt1959

But aside from that, there seems to be absolutely nothing on the internet about this chip. It's probably not a big surprise they don't want the datasheet to be freely downloadable for everyone, but given the age of the chip, I'm surprised that still nothing has leaked.

Or am I looking in the wrong places?

Are you planning to implement a new drive using the chip? Or new firmware for an existing drive?

I was just curious, how you would modify firmware or create a new one for an existing drive.

The "how" is dependent upon how the chip is installed. Beyond the "load this firmware" implemented in their boot code, you would need to know the hardware it's installed on. Fastest way would be to get a dump of the firmware installed. Fortunately, there are quick ways to do that, given that people have already dumped stock and modified versions of the firmware. Decoding it... is a separate issue.