i don't know all the details of how it happens, but it seems like NS pulls in any workflow actions which set the native approval status on JEs to that page automatically - i've never had to set it up, but when i have multiple approval workflows it pulls in multiple approval actions. as for setting up your own, if you want a more customized experience for the users, you could build a suitelet that performs a similar action (like the advanced approvals bundle did). it's mostly useful if your users are going to be bulk-approving JEs, though - i wouldn't recommend that type of view for any individual JE approvals since it doesn't allow you to view most of the data you would need to actually be fully reviewing a JE before approving it. but it can come in handy if you have like, 100 fixed asset JEs auto-created or something and don't want them to be automatically approved, but the user wants to bulk-approve them once they've confirmed from external sources that they're all correct, rather than clicking into each one 100 times.