Nope, I didn't use those.  That must be one of the reasons my definition of exchange suck.  What I have used, though, is currency exchange service, in person, where I pay in one currency and get the [buying or selling] value of another currency, i.e. BTC, CAD, EUR and USD.  I very much prefer a cash transaction over one tied to a financial institution account.

While they write "exchange" in bib letters, and show the "selling" and "buying" value of each currency they trade in, then I just reused the term buy and sell.  Then while they "had" the currency they gave me and that they took the currency I gave them.... then I assume there is a certain amount of those currency in their hand.  And by "hand", I mean possession.  And by possession I mean under their supervision.  And while it is under their supervision, I say they are holding it.

You are indeed talking about exchanges where users offer to sell or buy whatever we want to exchange, though I have a vague idea of how I believe that works.  I've seen some charts, heard about it, but never went very deep in the understanding of its backed.  I would assume that I still pay to the exchange and the exchange pay me, but I can be mistaking.  If my assumption is right, it would mean that they need to have a certain amount of what they trade in their hand, even if the currency, the stock or the bitcoin legally belongs to the user during the process.

So those offices that change your currency, are they exchanges?  Or how do you call them?
And do exchange need to hold any asset at any point?  Or how can the exchanged assets be moved between users?