There are many architecture patterns and you can follow any one that suits your need. By default android follow mvc but most developers prefer mvvm since its also recommended by google itself.

Since you are new to android I would suggest you to follow the default architecture (mvc) you can place the business logic in your activity or fragment (especially if your building a small app).

First get familiar with android eco system by making different small apps then when you are confident move to advance topics. (Its my opinion, others can agree or disagree).

Incase you prefer to follow other architecture patterns like mvvm then you can put your buisness logic in repository or use clean architecture with mvvm and place your business logic in usecases.