It looks like it's probably Studio 3, which isn't seen (on BT Sport at least) too often. The Goals Show is in 2 as usual for this season (the little mezzanine studio is even smaller than it looks on screen so not great for social distancing) and the Manchester United game is in Studio 1, again presentation not on site in a Tier 3 area.
They wouldn't have done presentation on site even if Manchester wasn't in Tier 3, pretty much all the group stage is presented from the studio (maybe they have done the odd really big game in the group stage at the ground, but very rarely), seemingly based on the idea that it's a big nerve centre that can oversee everything. Sky did the same when they had the Champions League, only going on site in the knockout stage. As mentioned, they were in the same studio for Man City last week, and for Liverpool on Tuesday.
Curious scheduling to put a game with no UK interest on BTS1 and Chelsea on BTS3
Not really, it's always been the case that the live Champions League games involving British sides are on 2 and 3, with the Goals Show on 1. By putting Chelsea on 3 as well, and a non-British tie on 1, it meant they could carry on with post-match analysis and interviews on 3 without having to dash off for the Goals Show if it was on 1.
Again, it was much like how Sky used to do it, the two English matches were on 2 and Xtra/4 for many years. They do need to offer stuff on all the channels or there's no point in having them all.
Funnily enough in the Europa League it's been the English sides on 2 and ESPN and the Scottish sides on 3, but tonight I see Leicester have been moved to 1 due to some unscheduled hockey on ESPN. But the "main" game is still on 2.