How do I access my diaphragm?

I’ve been practicing my vocal performances with a friend and they’ve been trying to explain that I need to use my diaphragm. How do I do that? I can’t visualise how to use my internal organs, the process is automatic.

As far as I can “feel”, noise starts at the voice box. I know lungs are involved but I don’t really understand how to use them in ways that aren’t just breathing.

How do I access it and like tap into being able to use it properly?

Everyone I’ve asked about this (usually has a lot of training musically, but also) seems to act like it’s something I should already know how to do.