The 3 tutorials apply to 3 different situations, so pick the one that applies to what you want.
Running Discourse with a separate PostgreSQL server is for when you have an external PostgreSQL running somewhere else, like AWS RDS.
Multisite configuration with Docker is about running multiple Discourse instances inside the same container.
And this topic is about using different containers for data and web.
The three guides are for advanced users, and we recommend sticking to defaults for people who aren’t familiar with Discourse, containers and the whole sysadmin lingo.