The vast majority of my switches and outlets registered as generic z-wave devices. I note that in the mobile app, I can search for the particular device, but when I go to the IDE to update the handlers, I don’t see device handlers for items listed in the app.
What am I missing?
I’m assuming That by “update the device handler” you mean to change the device handler that a specific device is assigned to? Not actually change the code of an existing device handler?
If that assumption is correct:
sign into the IDE and get to the list of your devices
click on the name of the device you want to reassign. That will open its details page.
go all the way down to the bottom of the details page and click the red EDIT button. That will take you to a page where you can assign the device to one of the other device handlers in your account’s library. 
If my assumption is incorrect, and you’re actually asking how to publish a new device handler to your account, see the custom code FAQ:
Your first assumption is correct, the problem is that I only have the generic device handlers available in the IDE. Maybe the handlers i’m looking for do not exist, but I’ve read forum posts discussing a Jasco/GE smart fan controller among others that I thought would be available in the IDE but are not.
They could be community created device handlers. There are many of those. To use one, you follow the instructions in the FAQ that I already linked to.
To find them, use the quick browse lists in the community – created wiki. They are organized by device class.
The only things that will be in youraccount library to begin with are the generic handlers and the ones on the official compatibility list.
Meanwhile, here’s a popular one for the GE fan control:
OK, I just checked the official compatibility list, and unless I missed it, the GE/Jasco fans switch isn’t on it. Only the Leviton fan switch. So there won’t be a GE fan switch device handler in your account library until you publish One to yourself using the instructions in the custom code FAQ.