How to Multi-Select
Hold Shift and Drag
Hold down Shift
Click down on mouse or trackpad
Drag across all the items you want to select
Release when everything you need is highlighted
This creates a selection box that captures everything in its path. From here you will see the option to copy the items, group them, and auto-layout.
What You Can Do with Multi-Select
Create Groups Instantly - Select multiple sources and press "G" to group them
Move Multiple Items - Drag them all at once to reorganize your board
Copy in Bulk - Select everything you need and hit Ctrl+C (or Cmd+C)
Delete Multiple Items - Select and press Delete to clean up your board fast
You will also see the option to auto-layout the content that organizes everything nicely so that things are not overlapping.