Select First, or Shift Marquee Selection Left    ⇧↖

Shift-Home. The Shift Right command is Shift-End.

Yes, the Marquee selection moves to the right or the left, but I cannot figure out what is used to determine how far to shift.

Mystery challenge of the day.

Select parts of regions in the Logic Pro Tracks area — Apple Support

Press Shift–Home (the default Select First, or Shift Marquee Selection Left key command).