Am I correct in thinking that you can't open up a map in Radient unless it comes with the .map file in the .pk3? I'm interested in viewing how other mappers built their maps - I think it's a good learning tool.

Nope u can't, there is a tool out there (I believe made for Quake 3) but you'll be lucky to get anything looking like the original map file from it.

Take a look over the tutorials and videos on this site to learn mapping, I learnt everything I know through this method.

No, you cannot open a bsp in radiant. Most people do not include their map files because they don't want anyone stealing them and most say so in the readme they include. MP_Foy for UO and Dawnville from CoD are included with their respective tools. You can look at them and see how they did things.

there are also a couple of other maps that have the .map included in the pk3

ubootbunker and shotguns rooms are 2 that i know of off hand

Farm2 has included all compile files.. The map file too Farm2 has 2 beautiful houses