How do Image Optimize pipelines coexist with core's Image toolkit optimization? I have the Image Optimize module installed. I've also installed and configured Image Optimize Binaries and Image Optimize reSmush.it.
I've created a simple pipeline that processes an image through reSmush.it's magic, setting the desired quality to 80%. This basic setup allows me to apply an optimization process to images after they have an image style applied. (The Image Optimize module adds an option to the image style creation form to choose your optimization pipeline.)
From my understanding, for any particular image, core's process might look like:

Original image uploaded via image field (1200x800px, unoptimized).
In my display, I apply the image style to scale and crop this image to 300x200px.
When this image is accessed, the image is scaled and cropped to 300x200px, and then core's JPEG quality would take effect and optimize this cropped image.

With the Image Optimize pipelines, I'm not sure if core's JPEG quality settings are still used: does the image get core's optimization plus the Image Optimize pipeline? or does the Image Optimize pipeline override core's optimization? I can't seem to find an answer to this anywhere. How do these options coexist?
With the Image Optimize pipelines, I'm not sure if core's JPEG quality
settings are still used: does the image get core's optimization plus
the Image Optimize pipeline?

Yes

does the Image Optimize pipeline override core's optimization?

No
According to the module's creator you should set your core's image toolkit to 100%; otherwise, you will endup with double compression and lower image quality.