I have a 2017 i3 and, like you, have found no way to add apps to my car. I've checked at https://connecteddrive.bmwusa.com/ and my car is subscribed to almost everything for another year, but if I try to visit the Connected Drive Store from my car, it says that it's not available or something like that. The normal BMW ConnectedDrive functions (like remote lock/unlock, location, climatize, charging monitoring, etc) all work fine from my phone, but I haven't found any way to add any third-party apps to the car's infotainment system.
The i3 was updated to the "NBT Evo" infotainment system in 2018 and later models from the "NBT" infotainment system in the 2017 and earlier cars. I don't know if there were ever third-party apps for the older NBT, but, if so, I can't find a way to install them at this point.
I can tell that there are folks on this forum who understand the ins and outs of NBT vs NBT Evo a lot better than I do, though. Perhaps someone else can chime in here as to what apps, if any, you can install on an NBT.