Testing in MVVM architecture. What to test and how to approach TDD?

I’m currently working on a personal Android project using Kotlin and Android Studio, following a basic MVVM architecture:

─main
│   ├───java
│   │   └───com
│   │       └───example
│   │           └───project_name
│   │               ├───data
│   │               ├───db
│   │               ├───repository
│   │               ├───ui
│   │               │   └───adapter
│   │               └───viewmodel

I have worked with Hexagonal architecture and Test-Driven Development (TDD) at work, which makes testing pretty straightforward. However, this is my first time doing Android/Kotlin and MVVM for a personal project. I also did some WPF with C# and LINQ before, but the structure is a bit different and, shamefully, I didn’t do much testing nor develop with a TDD approach back then.

My questions are:

• Which classes/components are the most important to test?
  I’m guessing the business logic is in the data layer. I also test the ViewModels. Is there anything else worth testing, or is it useless?

• Is it worth testing UI elements (buttons, adapters) with unit tests, or only with UI/integration tests?
  (Not even sure that I’m going to test UI.)

Also, if you wonder why I didn’t do Hexagonal architecture here, it’s because I unfortunately couldn’t get it to fit well in Android Studio, so I followed the basic MVVM pattern to have a working app I can actually use before I die 😆.

Any advice on how to effectively write tests and do TDD in Android MVVM projects is very welcome!

Thanks a lot!