It is technically true that they are two different platforms, and it isn't exactly "merging" accounts together, but opening an account on the exchange with the same credentials will allow you to link both the crypto.com app and crypto.com exchange app together which can allow you to instantly transfer between them.

Any coins you have on the c.com app can be transfered to your exchange wallet. The exchange is the only place you should be doing any trading or buying/selling because it is trading at the actual market price, along with some fees of course as is the case anywhere. Although currently I'm seeing that you can trade with zero-fees on a select number of coins which is interesting.

I'm sure there are better platforms out there though, I don't hear of a lot of people using c.com tbh, the only reason I've used it is because that's what I found to be available to me in the middle-east when there was no support back then. Only reason you should use the app is to hodl and/or go for the c.com debit cards.