Env art is one the most complex art things in a game because its very extensive, it covers a whole range of things.
Its also something everyone appears to understand differently, you asked how an env artist works together with a level designer but in a lot of companies they simply are the same person. Or sometimes they arent… every company defines the name differently, the situation is so bad I use 4 different titles on my portfolio just to bring across the point what I do
What an env artist foremost is tho is someone who uses and brings together all aspects of an environment. Modelling, architecture, texturing, composition, lighting, sound, atmosphere etc.
Its that combination that makes its so extensive and that also makes it an environment. A lot of people make the mistake of only modelling some objects and rendering them and then naming it an environment. Without lighting, atmosphere, sound etc you dont have an environment but a group of objects.
And its that kind of stuff companies would want to see on a portfolio. The combination of things. And also very important is real time material. Renders are nice but mean little. You need to show you can also get it run real time and smooth without any technical problems + you understand the correct workflow.
Making inidivual rooms or objects is pretty simple, bringing all the different things together and making sure it keeps working and working well is the harder part.
Look outside the modelling community for tutorials on it. Inside the game communities, such as the Half Life, Quake or Unreal communities.
Youre best bet is absolutely to use an engine such as HL2/Source indeed and practice in that, create levels for the community. They can help you a lot trough critiques.
Youre better off using the Unreal engine tho, its far more user friendly and easier to import things + pretty much no company uses the source engine where as every single big publisher got a UE license. There are more than 50 next gen games in the go using the UE currently