Mobile app makers who are tight on budget have looked to hybrid options to develop their app.
Popular Hybrid frameworks such as Cordova which uses HTML/CSS/Javascript and React Native which uses CSS/Javascript allow the developers to build their apps using one technology for both iOS and Android platforms. However, app makers and designers often overlook the UI design and user experience that is unique to both iOS and Android phones.
Web technology doesn’t mean you have the design freedom of the web. You still have to work within the size limits and native UX of a mobile device.
A single UI design may be sufficient for both iOS and Android. However, you have to be sure to make it agnostic to both platforms. YouTube and Facebook are good examples of apps that use an agnostic design for both iOS and Android platforms. This can require lots of research to ensure that your UI design is intuitive and easy to use.
Refrain from using iOS or Android UI designs for both platforms. Users are very familiar with their devices and are used to the UI that is distinct to their devices. By having a design that favors one over the other might confuse your users.
Best and simple option is to create 2 designs; one for Android and one for iOS. This may seem like it’s more work but it’s likely less than the research you have to do to make sure your agnostic design work.
Follow iOS ui design guidelines from Apple https://developer.apple.com/design/human-interface-guidelines/
Follow Android ui design guidelines from Google.