I have iphone12promax with 516gig how do I expand the partitions to use the space

I'm not sure I understand your question. You can't partition the SSD on an iPhone. What do you mean your phone only accesses a small portion?

The iPhone 12 pro max came in three sizes, 128Gb, 256Gb, and 512Gb. If your phone is reporting 128 or 256Gb, then that is the size you have, not 512Gb.