Learning Backend for Android Development

Hi, I’m a new android developer (Started learning Kotlin about 6 months ago) So I have some experience with Kotlin. I created basic apps, even a basic social media app with Kotlin and Firebase but I want to create some bigger apps(for example a basic clone of Uber Eats App). So I realized that Firebase is not enough and I should learn some backend technologies. I made a quick research and found what I need to learn: MongoDB, NodeJs and Mongoose. (If you have suggestions for other backend systems you can write them down.) So my main question is that: Is it hard to learn backend and use it with Kotlin? Are there any Full Stack Android developers among you? Where should I start, or should I start learning Backend? Also if you have suggestions for tutorials it would be cool if you share them. Thank you.