You don't need any app, although you could probably find dozens by searching 'comprehensible input' on YouTube. Comprehensible Input is achieved by watching and reading the content you enjoy in your target language and rely on context to understand and acquire it . I am a polyglot, and after mastering French, I transitioned to English around 2017. Now, enough people have told me that my English is decently good, so since January of this year, I am now learning Spanish by watching lets plays of indie games and reading tech news articles.

I will probably start watching cartoons too. Something like the early seasons of spongbob square pants would be great because each episode is self-contained and humorous.

Here are some tips:

1. Learn the bare minimum vocabulary needed to have a general understanding of what's being said. Words such as conjunctions (and, but, etc.), prepositions (under, above, in, out, etc.), adverbs (fast, often, very, etc.), interrogatives (how, when, who, etc.), important adjectives (colors, tall, short, etc.), and the 100 most used verbs (be, have, want, do,say, go etc.). The goal is to later acquire new words from context.

2. At first, you will not be able to understand new words from context alone most of the time , so you'll need to check the dictionary very often. Over time, you'll rely on context more and more.

3. Depending on the target language, you don't need to focus much on grammar, but don't omit it completely. It's easier to just learn why the sentence is the way it is, than to slowly memorize the correct structure.

4. Mastering a language is a long process. Depending on how far or close your target language is to your native one, it'll probably take you from 6 months to 2 years to acquire enough of the language where you'll unlock the full potential of passive learning and you'll slowly master your target language by just consuming the media of said language.