Depends on how PRN positions are allocated. In some organizations, all PRNs are in a centralized "department" and used like an internal float pool. There may be different categories of PRN- in terms of how many shifts/holidays you have to work. In others, each department may have their own PRN staff who only work on that unit. And other hospitals may have a combination of both.
But, in any type of arrangement - PRN positions are hired just like all other staff positions and your point of contact is HR. Jobs are advertised, and you have to go through the same application, hiring & orientation process. This is an important consideration - even if you only want to work a couple of shifts a month, you may have to commit to a lengthy orientation period.
If you're an experienced nurse and just looking for a few extra shifts, you may want to check out some of the PRN agencies in your area. That is a good way to 'try before you buy' to explore different facilities. You can also get some inside info on how they manage PRN staff at different facilities.