As a german I can confirm that in those kinds of schematics something like X1 is most likely some screw terminal block as connection point for the things after them, like buttons, sensors etc.
The number after the X (in this schematic, I see X1,X13,X20,X21,X28) would be the connection terminal itself, followed by the number of the screw where the wire goes to.
And also the wire colors are german stuff, like:
bl=blue
sw=black
gr=grey
gn=green
ws=white
rt=red
br=brown
I would read this part in a way, that:
From connection point X21 screw 3 goes a blue/black wire with 1.5 mm² size to Terminal block X1, Screw 2. From X1, screw 3 (here the first numbers probably are the incoming terminals, the other some lower row) there goes a white connector to the incoming connection point Screw 2. From X1 screw 3 then there is a black/white cable with 1.5 mm² that goes to Y3/screw 1.
A photo of the terminal block or connector wiring panel X1 would greatly help to confirm and explain some things, though ;-)