You can use Kafka or RabbitMQ - it is widely used as Message broker and it is not outdated - it is a question of popularity.
About comparison with Redis, please, take a look: https://www.educba.com/rabbitmq-vs-redis/
Hi @Vaibhav! From the perspective of your Flutter Mobile app Backend can be written using any technology: ASP.NET Web API, or Java or NodeJS, PHP, ect.
Here I should choose the tech stack, that you familiar with (or the simplest to start with). It can be ExpressJS or Node.js
As a database here you can use relational as PostgreSQL or Non-relational: MongoDB.
