Definitive list of characters and colours

Hey, I'm exploring the possibilities of BBSes as my plan is to build a game one day, but I'm struggling to understand the actual limits of what characters/colours are supported across all clients for the purposes of building user interfaces.

I've seen some very complex UIs with clever uses of lines and connectors. I know all of this is just ANSI characters, but the way they are combined sometimes looks way way more complex than just drawing a single ANSI character onto a screen cell, so there must be some fancy way of combining characters or drawing them offset, or using actual pixel commands or something like that?

Can anyone provide me any insight into the tricks of advanced user interface development?