To be sure your app will be validated by Apple you have some things to know (for Android’s Google Play there is no validation, so it’s more easy):

• All your pages have to be filled out with content. Please be sure you have no empty pages. If you have a news feature for example, enter one news, and with a picture it would even better.
• Content: no explicit contents in your apps! But it was obvious 😉
• Design: please choose cool colors with complementarity. If you don’t really know how to find colors with complementarity you can use https://colorschemedesigner.com/. But no rainbow apps, please don’t.
• Features: Apple want useful apps. So don’t create apps with only one or two features. And enter relevant content with multimedia (videos, images, etc)
• Write a good description of your app. You have to explain what the users would be able to find in your application, and why they have to download it. (250 characters minimum).
• Never talk about other platforms in your app! Apple doesn’t want you to talk about Android or Google Play, or Samsung in apps that are on the Apple App Store. Even if there is a logo, or an image about an Apple competitor, your app will be rejected.
• If there is an account registration tab in your app, or part of your app that is blocked with the Padlock feature, be sure to provide test credentials to Apple in the “Demo” fields on appstoreconnect.apple.com if we have published the app under your developer account, or to send these credentials to us prior to publish your app. To know how to create test account, take a look to the Padlock feature article.
• Never use test content in your app, like “Lorem Ipsum” content or “test”, “demo”, etc. Apple refuses apps that are not finished, and/or apps for tests.