Recovering purgeable memory on iPhone

No.

There is Purgeable Space and there is Empty Space.

The two are not one and the same thing

Purgeable Space which is Controlled by the Operating System.

When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur.

I assume that you are talking about storage space – not memory (RAM).

My understanding is that macOS will automatically delete things that it is storing in purgeable storage space if it needs the space for something else. But in the meantime, macOS might feel free to fill quite a bit of what would otherwise be free space with purgeable files.

I think you should explain. If you are asking about an iPhone, where are you finding this purgeable memory? Explain in steps someone else can follow.