Not getting app catalog purchase confirmations

We are just getting into the cloud, and hadn't yet turned on the app catalog, but then a new project started that required us to.

We purchased a license for 5 workflows from a large workflow vendor, and as part of that we gave them our tenant ID so they could authorize us to use their software.  We then had to turn on the app catalog, so the software could be downloaded to the site that will be using those workflows.

The interesting thing was that no request ever came in to the app catalog request thing.  I thought that we were supposed to get a notification anytime anyone wanted to 'purchase' an app, and then an admin would have to approve that request.  Was that because it was 'free'?  

We ended up shutting down the app catalog because we want to make sure we understand how it works.  I've searched all over on the internet, but the problem is I keep finding stuff I think is out of date. (6 months old is a long time ago for an o365 post).  

Can someone explain to me the following:

1.  If a user downloads a free app, is there any approval required?

2.  If a user downloads a paid app, is there any approval required?

3.  If there is a charge for an app, how is it paid?  Via a credit card?  A monthly bill from MS?  Bills directly from the app vendor?

4.  Who exactly approves app purchases?  Is it just the Global Admins?  Or is there a sub admin role like the license admins who can do that?

5.  Is there any way to restrict which users can download apps?

6.  Is there any way to have a message pop up notifying users of corporate/finance policies regarding app catalog purchases when they go to the app catalog?

We are afraid to turn this on and then wind up with hundreds of apps downloaded and then getting a big bill from app vendors for all the licenses downloaded.   Or users paying with their company CC thinking that if IS has allowed them to download these apps then they must be approved by corporate.

Thanks.

Ted