DVDs, DVD-As and Blueray disks can limit the digital output sample rate to 48kHz. It’s a part of the Digital Rights Management (DRM) that’s designed into them. Not all limit the output sample rate, but many popular ones do. (SACD has a similar limit: no raw DSD bits are allowed outside a unit.)
Even so setting an Oppo up correctly to get the maximum resolution on the digital outs isn’t simple. There are some posts around here that talk about it: Among other useful posts I remember some by tarnishedears in the “Getting Oppo DSD Output to the Direct Stream Dac” thread. Here’s one but perhaps reading more of the thread might be helpful.
https://forum.psaudio.com/t/getting-oppo-dsd-output-to-the-direct-stream-dac/2744/115 (you might need to scroll up to the top of that post.)
There isn’t a simple (passive or direct wire to wire) adapter from I2S to HDMI, there are various HDMI to I2S things with processors (or at least specialized chips) available, but, in general commercially built and sold ones won’t defeat the DRM mentioned above - HDMI was designed with DRM in mind.