>  Is it?

Yes.

> What type of computer building related questions can be asked here

Any that isn't specifically excluded or covered elsewhere. The only relevant exclusion in this case would be asking for hardware recommendations, which is covered on hardwarerecs.se

> how will beginning users know that is that type of site, seeing as how it's not marketed/advertised that way?

The [What topics can I ask about here](http://superuser.com/help/on-topic) page makes that pretty obvious, methinks:

> Super User is for computer enthusiasts and power users. If you have a
> question about …
> 
> - **computer hardware**
> - computer software, or 
> - personal and home computer networking

Computer building is the art of assembling of computer hardware, and since it isn't excluded later on, it's on-topic.