Both TRS and XLR inputs share the same signal path. If the diagram is correct (there is at least one obvious mistake on it), the XLR input signals actually go through the TRS contact switches before reaching the preamp chip, meaning that inserting a TRS plug breaks the connections from the XLR jack. Not sure why they'd wire it that way, but you'll be well advised to keep the TRS portion of the combo jacks clean because of that.
TRS 1 DET (and TRS 2 DET) seem likely to indicate TRS insert detection switch lines. Probably used to switch input gain from line to mic level if that's done automatically by which input is used rather than by the user. The 165 box is not a component but a connection flag which indicates that signal connection to another diagram or another point on this one.
I need some help determining if the DR-100mkIII line-in input actually bypasses the mic inputs.
Why?