The attribution must be visible to the end-user.
For websites, you can attribute a Lottie animation and credit the original designer anywhere on your site.
When it comes to attribution for an animation in an app, you can give credit anywhere within the app (for example, you may have a small section where all your asset credits are) or even in your app store description.
Here the user has given attribution beneath the animation itself.
Here the user has given attribution at the bottom of their webpage.
Note: Animations bought via the LottieFiles Marketplace do not require attribution.