HomeAssistant on Raspberry Pi/ SD-card image version

HI all
Sorry for a beginners question.
Last year I installed your image to a SDcard and put that into a Raspberry PI.
I could not fine much use for it in that configuration.
Is that correct to assume that I cannot use that raspberry pi for anything else?
For example, if I want to use a mqtt-server for handling some lights and so on, that mqtt server must reside on another computer?

I think if you take a look at Add-Ons your questions will be answered.

I was thinking more general, things that do not exist in Add-on

BTW, I have no supervisor menu, so I cannot add Add-ons. Iam using a docker instance on a synology NAS. version 202-12 I think it was called. add ons is not supported on docker, maybe? And I am the only user on the system, and therefore the “owner” or root it it had been a Unix-system. Also, even being the Owner I cannot create other users.

In the initial post you say you use a Pi and now suddenly it’s a synology NAS.

Take a look here for an overview of the different install methods.