Glazing beads are cut to size for each window sash and fitted exactly to mitre. This can be done by sawing or, in the case of narrow profiles, by punching.
Mitre saw
There are simple mitre saws, each one working on one end of the glazing bead. This can be a manually operated machine or an automatic machine. However, there are also double mitre saws that cut both ends at the same time and are both faster and more precise.