The new interface doesn't appear to show DLC at all in the game information, where it used to say "+3 DLC" or whatever, and also show ones that are available which you don't have installed with an "install" button next to them. There is no reference to DLC anywhere in 2.0 that I can find, other than the backup installers under "extras".
But I just got the DLC for Pathfinder - Kingmaker, and it didn't appear anywhere in he game info. So I tried manually updating the game. Then when I played it, the scenarios both say that I have not purchased them. I guess I'll download the backup manual installers, but this seems to be an important feature missing from 2.0 right now.

It also doesn't seem to show what version of a game you have installed anymore, so you can install patches if necessary.

game Dlc is in the "manage installation> configure"... press the square next to {on the right} the play button
non game Dlc i.e, soundtracks and other odds are ONLY in your web browser ui & yes that is a major pain

by default; new Dlc will auto install in Galaxy 2... assuming you don't turn that option off but its on by default
so for example you play Stellaris and the new Fed Dlc comes out so you pay for it... the next time you open Galaxy 2 it will see that you got the new Dlc and install it

in Galaxy 1 you have to click on your Fed Dlc box but not Galaxy 2

p.s, yes offline installers are a different thing to Galaxy installers... in most games its one or the other mate