There are many things missing/working differently/placed somewhere else on the mobile (portrait UI) version of Roon. Mostly because of (un)available screen space. I’m pretty sure that the extensions tab is missing for that very reason. The current portrait UI is a nice supplemental for playback control but it is not (and may never be) a full featured control option for Roon.
Note: Even the tablet (landscape mobile) version of Roon is missing a few features compared to the desktop version.