How come all expense Purchase orders are being auto approved? Can i disable that?

setup>accounting>accounting preferences>approval routing

its currently turned off for Purchase Orders

That means it gets approved up on creation. You do need to use an workflow to have a proper approval routing and process. There are few out of the box templates for workflow for approvals, unless you want to create a custom one.

we already have a custom one. So if i turn Purchase Orders on, everything will go through an approval process , is that correcT/

yup it will