I would consider a game called "Kandy Krush" sketchy if I thought that was really "Candy Crush", but it may still be allowed and it would be up to me to do the things like check reviews, check ratings, check Developer history, check for update frequency to gauge support, check their Privacy Page in the App Store, check in the App Store for the data that the app collects, etc.
As for submissions to the App Store, they must follow the App Submission Guidelines and will be removed when brought to their attention using the reportaproblem website. Certainly I have downloaded apps that I considered useless for me, but may still comply with the App Submission Guidelines. I delete those apps from my device and look for another one.
The checks performed by Apple on the submissions are automated checks for compliance, API use, Malware, like stated previously, each one is not being reviewed in person for any form of recommendation. You will find some apps on the Home page of the App Store that Apple may recommend as something you may like, but I have know idea how those are determined, or there may be a section for top downloaded app, which are exactly that.
As for payments within the apps, there was a time when Apple removed any app that directed a user to a third party payment processor and bypassed Apple's ability to manage the subscription/purchase. That was ruled illegal by the courts and Apple was fined heavily for doing that. When Apple attempted to warn users that they were being directed to an outside payment processor, they were again told to remove that warning. Use those at your own risk and any fraudulent charge would have to be dealt with the developer and disputed with the bank of the card being billed. Turning over your payment information to a third party also runs the risk that your card could be compromised.
And similarly there are widespread ads (or scams) on Social Media sites that direct users to a web page for them to start a free trial with the Developer directly and they also may have an app in the App Store. Even though they have an app in the App Store, that app may be fine when using the in-app purchases and comply with all of the Submission Guidelines, but you may still find that your interaction on their website may not have been truthful for any offer promised along with the payments and that is outside the scope of the App Store and how the app performs when using that platform.
