I know what's docker and why is it used for, but for example, if I have an app with the following setup: React + Rails + MongoDB + ElacticSearch + more things :)
Should I place each of those things inside a container or what? Because I'm a little bit confused in that part
Since you add tag DevOps on this question.
For production environment, I would recommend to take out of MongoDB and ElasticSearch from docker containers, which are easily managed by cloud providers seperately and get better performance.
You should be fine to manage React + Rails with docker or other docker orchestration tools.
For testing or non-production environments, you can do all in docker.
I would suggest writing a docker-compose file where you will define all the services & which eventually will be launched in separate containers.