If you're staying in Arc 2000, then you need to keep your evening's entertainments to places in Arc 1950 or Arc 2000. You can walk between the two of them.
From Arc 1800, you can (with great care + some torches + the odd bit of walking uphill) get down to Arc 1600 after the lifts close. You can't get any higher.
From Arc 2000, you can also get (with care and torches and the odd bit of uphill walking) get to Arc 1600, and also over to Villaroger. Having done the latter once after they closed the Lanchette lift early, I wouldn't fancy doing it after drinks or a large meal, so best not plan on it!
The odd chalet/resort might have a bus in which they can collect you from other resorts, but generally it's only the very expensive places that I've known offer it. Otherwise, your only option to get back from Arc 1800 to Arc 2000 is a taxi. I believe you'd be looking at something like €30-€40, based on the prices I've heard from people needing to get from Arc 1800 to Villaroger (further than 2000).
Arc 1950/2000 is quite a nice resort, but it's not the best place to stay if you want loads of easy apres-ski options. Other resorts in the area may do you better if that's what you're after!